Branch specific concurrency gates

[dliappis]

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):

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @dliappis? 🙏.
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-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Duration: 13 min 12 sec

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