Migrate to GitHub Actions

[rnro]

Migrate CI to use GitHub Actions.

Motivation:

To migrate to GitHub actions and centralised infrastructure.

Modifications:

Changes of note:

• Disable warnings-as-errors until issues with recent NIO changes are resolved
• Adopt swift-format using rules from SwiftNIO
• Remove scripts which are no longer needed

Result:

Feature parity with old CI minus allocation tests which remain on the old CI.