chore(ci): add mergegate configuration [backport 2.21] (#13804)

Backport 29a8a47 from #13785 to 2.21.

Add mergegate configuration to allow auto-generated PRs without commit
signing

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Co-authored-by: Quinna Halim <[email protected]>

Author: github-actions[bot]

repository.datadog.yml

@@ -6,4 +6,12 @@ workflow_type: noqueue # TODO: change to speculative once stable
 merge_method: squash
 gitlab_check_enable: true
 gitlab_jobs_retry_enable: true
-gitlab_fail_fast: true
+gitlab_fail_fast: true
+
+---
+schema-version: v1
+kind: mergegate
+rules:
+  - require: commit-signatures
+    excluded_emails:
+      - '41898282+github-actions[bot]@users.noreply.github.com'