chore(CI): improve milestone handling

[marstamm]

Proposed Changes

Following the discussion in https://github.com/camunda/camunda-modeler/pull/4400#issuecomment-2191806614 , let's keep closing the current milestone an explicit step in the release.

This allows us to merge and close issue in the time between Release build and Closing of the release issue.

Checklist

To ensure you provided everything we need to look at your PR:

• [x] Brief textual description of the changes present
• [ ] Visual demo attached
• [ ] Steps to try out present, i.e. using the @bpmn-io/sr tool
• [x] Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[nikku]

Verified that automatic milestone creation works.

What would be awesome is if we could attach the description Tracks the v5.{NEXT_MINOR}.0 release of the app. to it, cf. milestones.

I added that manually this time.

[marstamm]

We can follow-up to automate more things, like the description 👍

[nikku]

Cf. follow-up issue.