[7.17](backport #39298) Branch specific concurrency gates

[mergify]

Proposed commit message

PR #39293 introduced one concurrency queue per staging/snapshot but this slows down unnecessarily concurrent DRA builds for main and other release branches.

This commit makes the concurrency gates specific per branch.

Related issues

• https://github.com/elastic/ingest-dev/issues/3095

Logs

Example run: https://buildkite.com/elastic/beats-packaging-pipeline/builds/99#018f2ed1-88f4-4f2a-9604-e29c51eba3a3

Screenshots

First run:

Blocked second run (as expected):

---

This is an automatic backport of pull request #39298 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 40c68cf has failed:

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[elasticmachine]

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• Duration: 14 min 49 sec

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @dliappis? 🙏

[dliappis]

Closing as it's already done in #39197 (comment)