branching normalization

[baywet]

in an effort to streamline weekly releases:

• make main the default branch
• remove dev branch from workflows and github policy (PR to main)
• remove feat/3.0 from workflows and github policy (PR to main)
• remove feature/kiota-preview from workflows and github policy (PR to main)
• update weekly generation pipeline to target
• delete the 3 branches mentionned above once the PR has been merged

[baywet]

To complete the branch deletion we need #1533 to be merged so the policies are re-evaluated