Doc: Logstash 8.15.1 release docs

[karenzone]

Logstash release manager: @kaisecheng Docs release manager: @lcawl Logstash tech writer: @karenzone

• For reviews: @robbavey
• For doc questions, @georgewallace

Logstash release docs

• [x] Release notes: https://github.com/elastic/logstash/pull/16405
  • [x] Add link to docs release issue: https://github.com/elastic/dev/issues/2711
  • [x] Verify that release notes PR is forwardported to main: https://github.com/elastic/logstash/pull/16427
• [x] Update plugin docs:
  • [x] https://github.com/elastic/logstash-docs/pull/1716
  • [x] https://github.com/elastic/logstash-docs/pull/1720
  • [x] Bump lock file: https://github.com/elastic/logstash/pull/16401

[karenzone]

The Logstash team has the release process down to a science, and @kaisecheng is a seasoned pro. Logstash releases always go smoothly, and the purpose of this issue is documentation and connection.

Notes:

• I fill in the Docs release manager's github ID so that they will be @mentioned on this issue for awareness.
• As Logstash release manager, @kaisecheng will generate and curate the release notes PR. @robbavey will approve.
• When we get the PR number, I add the link and the Logstash release manager's github ID to the docs release issue. @georgewallace, please fill those in.
• Everything else is "nice to have."
• @kaisecheng will merge release notes by end of day on the day before the release. (She always does.) <3
• Reach out to @georgewallace if you need additional help.