bk: support DRA for branches that are not feature/*

[v1v]

Motivation/summary

DRA should run for any branch and be clever enough to know what DRA branch to be used, that's ok for PRs, but it's not the case for feature branches that start with a different name than features/

Fixes

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

See https://buildkite.com/elastic/apm-server-package/builds/6196

Related issues

[github-actions]

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