ci: filter mergify backports for the Exhaustive tests pipeline

[v1v]

Release notes

What does this PR do?

Filter the backports from mergify that use a different branch name mergify/bp/<branch>/pr-<number>

Why is it important/What is the impact to the user?

Exhausted tests don't run for backports. These should only be run when merged into a mainline stream

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 (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

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[elastic-sonarqube]

Quality Gate passed

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[elasticmachine]

💚 Build Succeeded

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• Commit: 3ba74c8

[donoghuc]

Regex looks solid and matches what we talked about on slack. Thanks!