docs: fix wrong PR links and other mistakes in the release notes

[pjfanning]

Motivation

Checking every PR and issue reference in the release notes against
GitHub, and the dependency versions against the release tags, found
wrong PR numbers, mismatched labels and URLs, two wrong dependency
entries, inconsistent heading levels and some typos.

Modification

• Wrong PR numbers:
  • 1.1.3 mixed Akka/Pekko cluster: PR1462 ("publish 1.0.3 docs") -> PR1562
  • 1.1.2 OSGi metadata: PR1464 (the sharding revert) -> PR1480
  • 1.1.0 virtual thread dispatcher: PR1299 (closed, not merged) -> PR1436
  • 1.2.0 StreamTestKit duration timeout: PR1648 -> PR1468
  • 1.2.0 CompletionStages helper: 2049 is the issue -> PR2060
  • 1.3.0 PR2336 and PR2337 were swapped
  • 1.0.3 TcpDnsClient: 1182 is the issue -> PR1183
  • 1.7.0 EndpointReader: fix: harden EndpointReader against NonFatal dispatch errors and unwrap WrappedMessage in writer logs (backport #3166) #3169 is a PR, link it as one
• Fix PR labels with issues/ URLs and vice versa (PR1743, PR1594, Cluster sharding delivers message to the wrong entity #1463)
• 1.2.0 dependencies: slfj4 -> slf4j; jupiter-junit 5.13.3 ->
  junit-jupiter 5.13.4 (per the v1.2.0 tag)
• 2.0.0-M3: "Bug fix" -> "Bug Fixes" heading, "Var Handles" ->
  "VarHandles"
• Version headings in releases-1.2 to 1.7 are H2 like the 1.0, 1.1 and
  2.0 pages (they were H1, the same level as the page title)
• Add the missing 1.1.1 milestone link, name the 1.2.1 milestone, and
  add the Java support line to the 1.0.x page
• Typos: "This is a milestone release", "to get it to work", "it is
  likely", an unfinished sentence and a stray parenthesis in 1.0

Result

Release note links point to the right PRs and issues, and the pages are
consistent.

Tests

• Not run - docs only

References

None - found during a review of the release notes

[He-Pin]

lgtm