automate more of the release process

[jamadeo]

This automates release branching using PRs. It uses github actions to create PRs that, on merge, create release tags. It also includes a check on these PRs so that, when targeting main (for a minor release) we check that each change exists in main's history (has been cherry-picked).

The process looks like this:

flowchart TD
    A[Weekly Schedule] --> B[Create Minor Release PR<br/>based on main]
    C[Manual Trigger] --> B
    D[Release PR Merged] --> E[Create Release Tag]
    E --> F[Create New Patch Release PR<br/>based on new release tag]
    
    B --> G{PR Status?}
    F --> H{PR Status?}
    
    G -->|Closed| I[No Action]
    G -->|Merged| D
    
    H -->|Closed| J[No Action]
    H -->|Merged| E
    
    style J fill:#ffcdd2
    
    style A fill:#e1f5fe
    style C fill:#e1f5fe
    style D fill:#c8e6c9
    style I fill:#ffcdd2
    style J fill:#ffcdd2