Remove CI runs on beta channels

[grtlr]

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md, if my changes are significant enough

[thibault-martinez]

Maybe you can reduce no_std to only stable, I don't think we gain anything from beta/nightly