main: update branch diagram in CONTRIBUTING.md

[ralfhandl]

Adjust branch diagram to refined release process

• we can't merge vX.Y-dev back into dev: this would mess up schemas and schema tests because
  • the openapi field requires pattern X.Y.Z
  • added fixed fields are reflected in schemas and schema tests and can't go into vX.(Y-1)-dev etc.
• add sync from main to dev and dev to vX.Y-dev

Tick one of the following options:

• schema changes are included in this pull request
• schema changes are needed for this pull request but not done yet
• no schema changes are needed for this pull request

[handrews]

Thanks for keeping this up to date!

I've added a suggestion to clarify what is going on with the new lines. If accepted, this would need to be done for the other change groups in the diagram as well.