Update README examples to use setup-micromamba v3

[Copilot]

The README still documented mamba-org/setup-micromamba@v2 even though v3 is the current supported major version. This updates the examples so the documented usage matches the version users should adopt.

• README examples

  • Replaced all documented mamba-org/setup-micromamba@v2 references with @v3
  • Covered usage snippets across environment creation, shell initialization, caching, debugging, and self-hosted runner examples

• Documentation correction

  • Fixed the malformed debug example that referenced mamba-org/setup-micromamba2v2
  • Standardized that snippet to the same @v3 form as the rest of the README

• Example

  - uses: mamba-org/setup-micromamba@v3
    with:
      environment-file: environment.yml
      init-shell: >-
        bash
        powershell