ci(buildkite): exclude files/folders that are not tested in Buildkite

[v1v]

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

[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @v1v? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

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

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.

[rowlandgeoff]

👍