ci: auto-label release PRs with release-management

[Maffooch]

Summary

• Extend the three Create Pull Request steps in .github/workflows/release-1-create-pr.yml and .github/workflows/release-3-master-into-dev.yml to await github.rest.pulls.create(...) and immediately call github.rest.issues.addLabels(...) with ['release-management'].
• Keeps labeling local to the release workflows rather than routing through .github/labeler.yml, so the label isn't subject to sync-labels: true re-evaluation on later PR events.

Prerequisite

The release-management label must exist in the repo before the next release workflow runs — issues.addLabels does not create missing labels. Create it once via the Labels UI or:

gh label create release-management --description "Automated release-train PR" --color 0E8A16 --repo DefectDojo/django-DefectDojo

Test plan

• Confirm the release-management label exists in the repo.
• On the next Release-1 run, verify the resulting PR carries the release-management label and the Create Pull Request step is green in the Actions log.
• Same check after Release-3: both the …into dev and …into bugfix PRs should get the label.

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