Remove unused gradle tasks

[donoghuc]

Release notes

[rn:skip]

What does this PR do?

This PR removes unused gradle tasks. It also removes a script from CI that is unused.

Closes #18476

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)
• /run exhaustive tests : Run the exhaustive tests Buildkite pipeline.

[mergify]

This pull request does not have a backport label. Could you fix it @donoghuc? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[donoghuc]

run exhaustive tests

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 5691426

[donoghuc]

This (simplified) graphic may help in reviewing this. Essentially it tries to map out where in our automation different tasks are used and the subsequent interplay between gradle and rake.

[donoghuc]

Getting the entrypoints mainly consolidated to gradle makes cleaning the unused gradle tasks quite straightforward. Going through the rake tasks will be a bit more complex as there appears to be some tasks that may be relevant to non-ci workflows. I am planning on separating out the rake task cleanup into a separate PR.

[mashhurs]

Docs have been changing alot, I might be out of sync.
Does this mean we no longer use reference docs workflow? - https://github.com/elastic/logstash-docs/blob/89fac69db8de43a1fca62aa1c4c983455d5986fd/.github/workflows/reference_docs-pre90.yml#L60