docs: update Hilla Gradle reference for Vaadin 25

[peholmst]

Follow-up to #6364, which updated the other Gradle pages for Vaadin 25 and left this page out on purpose. Hilla docs are being migrated, so this PR only corrects facts and doesn't restructure the page.

Fixes in articles/hilla/reference/gradle.adoc

Version and build file fixes

• Requirements: JDK 17 changed to JDK 21 or later, and Gradle now says 8.14 or later. Verified against articles/compatibility.adoc and GRADLE_MINIMUM_SUPPORTED_VERSION = "8.14" in vaadin/flow FlowPlugin.kt, which is the same on the 25.0, 25.3, and main branches.
• Node.js: "Node 18" is replaced by "Node.js (optional)", plus a sentence saying that the Vaadin Gradle plugin installs Node.js if it's missing or too old, with a link to Supported Technologies. The page doesn't give a Node.js version number because the minimum changes within Vaadin 25: FrontendTools on Flow 25.3 requires 24.0, and on main (25.4) it requires 26.4. This follows the wording docs: update outdated Gradle information for Vaadin 25 #6364 uses on the other Gradle pages.
• Requirements tag: nothing includes tag::requirements[]. Grepping the repo for tag=requirements and tags=requirements finds nothing, so changing the list affects only this page.
• Starter ZIP: the link now points to the v25 branch instead of v24. gh api repos/vaadin/skeleton-starter-hilla-react-gradle/branches --paginate lists v2, v24, v25, and main. v25 is the default branch and is current (Vaadin 25.3.1), while main is stale (Hilla 2.1). The archive URL resolves (302 to codeload).
• Build file examples (the main one and the pre-release one): org.springframework.boot changed from 3.0.6 to {spring-boot-version}, and io.spring.dependency-management from 1.1.0 to 1.1.7. The page now includes _vaadin-version.adoc, the same way articles/flow/configuration/gradle.adoc does. The v25 starter's build.gradle uses Spring Boot 4.1.0 and dependency-management 1.1.7, and 1.1.7 is the latest release in Maven Central's metadata for the plugin.
• vaadin-dev exclusion removed: the example no longer excludes vaadin-dev from vaadin-spring-boot-starter. That dependency isn't transitive in Vaadin 25: the 25.3.1 POMs of vaadin-spring-boot-starter and vaadin-core-internal don't list it. This now matches the v25 starter's build.gradle.
• gradle.properties: "set the Hilla version" now says "set the Vaadin version", and the example uses vaadinVersion={vaadin-version} instead of a hard-coded 25.0.0, as in docs: update outdated Gradle information for Vaadin 25 #6364.
• Project tree: gradle.build changed to build.gradle.

Hilla plugin fixes (found while comparing the page with the plugin source)

• exposedPackagesToParser removed: the hilla { exposedPackagesToParser = ... } option and its note are gone. EngineProjectExtension, which defined the hilla extension, was removed from vaadin/hilla in 24.7, and the option doesn't exist on any 25.x branch. A build file using it would fail. In its place, a short paragraph says that services in dependencies and other modules need no build configuration, and links to the Multi-Module section of the Hilla Configuration page and to the Explicit Discovery section on this page. That Configuration section is where the Maven docs already cover this.
• hillaConfigure: removed the claim that the task writes build/hilla-engine-configuration.json. In 25.x, EngineConfigureTask only calls EngineAutoConfiguration.setDefault(...). For the same reason, the hillaGenerate description no longer says it "reads the configuration file".
• Plugin name: "Hilla Gradle plugin" now says "Vaadin Gradle plugin", and "Hilla pre-release versions" now says "Vaadin pre-release versions". Users apply com.vaadin, which vaadin/platform builds from the Hilla plugin sources together with flow-gradle-plugin.
• Link to the plugin's other tasks: the note about vaadinPrepareFrontend and vaadinBuildFrontend now links to Gradle Configuration Properties. The Getting Started page it linked to doesn't describe those tasks.

Verification

• vale articles/hilla/reference/gradle.adoc: 0 errors and 0 warnings. Before this change, there was 1 error (Gradle Plugin in a block title) and 2 warnings (AOT undefined), and this PR fixes them.
• Rendered the page locally with Asciidoctor. {vaadin-version} and {spring-boot-version} resolve inside the subs="normal" blocks, and the new xrefs resolve to compatibility, flow/configuration/gradle, configuration, and #_endpoint_discovery.
• articles/hilla/lit is untouched. No page moves, so no redirect is needed.

Not changed

• The project tree still shows themes/ and views/helloworld/HelloWorldView.tsx, while the v25 starter uses views/@index.tsx and views/@layout.tsx and has no themes/ folder. Updating it would also mean rewriting the Hilla Lit note under it, which goes beyond correcting facts.
• The WAR packaging example uses providedRuntime 'org.springframework.boot:spring-boot-starter-tomcat', while the Spring Boot 4.1 docs now recommend spring-boot-starter-tomcat-runtime. articles/flow/configuration/gradle.adoc has the same line, so both should be changed together.

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• hilla/reference/gradle — modified, +26/-37 lines, 18 removal marker(s)

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