chore(docs): update release template

[jarekdanielak]

Proposed Changes

Make sure code freeze is announced after the release manager is assigned.

We do it, but it's not documented.

Checklist

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