Migrate to GitHub Actions

[rnro]

Migrate CI to use GitHub Actions.

Motivation:

To migrate to GitHub actions and centralised infrastructure.

Modifications:

Changes of note:

• Raise minimum Swift version to 5.9
• Migrate some benchmarks to swift-service-context
• Run benchmarks in CI
• Adopt swift-format using rules from SwiftNIO
• Remove scripts and docker files which are no longer needed

Result:

GitHub Actions CI in parity with old CI plus benchmarks CI.

[ktoso]

updated branch protection rules