Improve GitHub Actions workflow

[ebkalderon]

Changed

• Use consistent YAML array indentation.
• Trigger main workflow on release/* branches as well.
• Collapse the cargo-test-{tokio,agnostic} jobs into a single job and build matrix.
• Replace custom cargo-audit job with actions-rs/audit-check.
• Enforce stable clippy and rustfmt.
• Rename main.yml workflow to ci.yml.

Removed

• Remove unnecessary empty needs keys.
• Remove redundant comments.

Follow-up to #309.