docs(15.9): move the install and upgrade guides to OpenSearch 3.9.0 and document the reindex an upgraded index needs

[marevol]

Summary

Fess 15.9 now runs on OpenSearch 3.9.0 (codelibs/fess#3537), and bin/fess-setup install opensearch installs 3.9.0, but the 15.9 pages still told readers to set up OpenSearch 3.8.0 and its 3.8.0 plugins. This PR moves the 15.9 tree to 3.9.0 and adds the reindex step an index created by 15.8 needs. All seven languages (ja, en, de, es, fr, ko, zh-cn) are changed the same way.

OpenSearch 3.9.0

• The OpenSearch version Fess 15.9 uses is now 3.9.0 in install, install-linux, install-windows, prerequisites, run, troubleshooting, uninstall and upgrade. This covers download URLs, paths, the fess-opensearch image tag and the supported-version statements.
• Two kinds of sentence still say 3.8.0 on purpose:
  • "OpenSearch 3.8.0 and later refuse dictionaries outside the configuration directory": the behaviour started in 3.8.0.
  • The configsync 3.8.0 note in admin-index-backup: it describes older plugin versions.
• The plugin install commands now use the URL form. Since 3.8.1, opensearch-analysis-fess, opensearch-analysis-extension, opensearch-minhash and opensearch-configsync are published only on maven.codelibs.org. opensearch-plugin install resolves the groupId:artifactId:version form against Maven Central only, so that form fails for 3.9.0.
  • Checked: the 3.9.0 zips return 200 on maven.codelibs.org and 404 on Maven Central, where the latest version is 3.8.0.
  • The commands now use the maven.codelibs.org URL, as the docker-fess opensearch/3.9 image does, and each command block says why.
• Step 4 of the upgrade guide now notes that a snapshot taken on 3.9.0 cannot be restored to 3.8.0 or earlier.

Reindexing a document index created by 15.8

At startup, 15.9 adds missing fields to the configuration, log and user indices only (codelibs/fess#3517). It leaves the document index alone, so an index created by 15.8 does not get two 15.9 changes:

• Owner fields (#3528): owner and last_modifier are mapped dynamically as text. As a result, owner:alice and facet.field=owner do not work.
• Kana normalization (#3532): the normalization of kana variants and of dashes written as long vowels lives in the index analysis settings and in mapping.txt, so the existing index never receives it.

The new upgrade section "Applying New Fields and Kana Normalization to an Existing Document Index" says:

• To apply both without crawling again, run Re-indexing with Replace Aliases and Reset Dictionaries on the Maintenance page.
• Reset Dictionaries overwrites the dictionaries with the bundled files. That is how SearchEngineClient#createIndex uploads them. Anyone who edited synonyms or other dictionaries should download them first and apply them again afterwards.
• Files indexed before the upgrade have no owner until they are fetched again. Incremental crawling skips unchanged files, so this needs one crawl with Check Last Modified turned off.

Step 7 and the "Do I need to recreate the index?" FAQ now link to this section. The option names follow the admin labels (labels.upgrade_reindex, labels.replace_aliases, labels.reset_dictionaries, labels.incremental_crawling) in each locale.

Verification

• Plugin versions: the 3.9.0 zips of the four plugins return 200 on maven.codelibs.org. The OpenSearch 3.9.0 tar.gz, rpm, deb and Windows zip return 200 on artifacts.opensearch.org.
• No regressions found by docutils: I parsed every changed file with docutils and compared it to main. Section structure, the number of system messages, and the counts of leftover raw `` and |Fess| are unchanged.
• Cross-references: a minimal Sphinx build of each language's upgrade.rst resolves the new upgrade-reindex-new-fields label with no new warnings.
• Behaviour: checked on a Fess 15.9 build on OpenSearch 3.9.0, using an index carried over from 15.8. Owner search and kana normalization did not work before Re-indexing with Reset Dictionaries and worked after it. An incremental crawl did not add owners to unchanged files; with Check Last Modified turned off, every file was fetched again.