Trigger dependency build-related jobs on release branches too

[alopezz]

What does this PR do?

It does the equivalent to what is currently already done in master w.r.t. dependency builds, uploads and lockfile updates to release branches.

Motivation

With the new dependency build process, there's no good way to cherry pick individual dependency changes to release branches (or to update individual dependencies within a release branch, for that matter). This is because lockfiles are resolved for a predefined set of direct dependencies (as defined in agent_requirements.in); cherry picking a dependency update out of many updates would result in a new set of direct dependencies that would need separate resolution.

Additional Notes

The auto-PR should, according to the documentation for the Action that we use to generate it, inherit the base branch from the PR that triggered the workflow, which is what we want.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Changelog entries must be created for modifications to shipped code
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged