[8.x](backport #42404) ci(buildkite): exclude files/folders that are not tested in Buildkite

[mergify]

Proposed commit message

Run faster builds and avoid waste of CI cycles.

I decided to support the whole .github folder since some other files such as CODEOWNERS or .dependaot.yml and the rest are not actually used by Buildkite.

I took some of the files that we skipped in elastic/elastic-agent#4836. I don't know the reason it was not configured in this repository, too, but I think it's the same case.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

Related issues

Similarly done in elastic/elastic-agent#6579

Use cases

Screenshots

Logs

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This is an automatic backport of pull request #42404 done by [Mergify](https://mergify.com).

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