Doc: Logstash 8.15.2 release docs

[karenzone]

Logstash release manager: @andsel
Logstash tech writer: @karenzone
Docs release manager: @bmorelli25

Logstash release docs

• Release notes: Release notes for 8.15.2 #16471
  • Add link to docs release issue: https://github.com/elastic/dev/issues/2774
  • Verify that release notes PR is forwardported to main:
  • Verify that release notes PR is forwardported to 8.x: Backport PR #16471 to 8.x: Release notes for 8.15.2 #16478
• Update plugin docs: updated docs for 8.15 logstash-docs#1733
  • Bump lock file: bump lock file for 8.15 #16454

[karenzone]

@karenzone: ToDo: Backport strategy for 8.x release notes to main. Keep updated? Abandon what's there until 9.0.0 feature freeze? Delete what's there?