[9.2] (backport #11414) ci: use if_changed

[mergify]

What does this PR do?

remove dependency on the BK plugin monorepo-diff

Why is it important?

Faster builds
Remove the intermediate step to load the monorepo diff step

Checklist

• I have read and understood the pull request guidelines of this project.
• 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/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

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This is an automatic backport of pull request #11414 done by [Mergify](https://mergify.com).

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 291cc46

Failed CI Steps

• x86_64:sudo: container - platform-ingest-elastic-agent-debian-13-1762801856

cc @v1v