Drop support for Swift 5.8

[simonjbeaumont]

Motivation:

Now that Swift 6.0 has been released we can drop support for Swift 5.8, as per our policy to support the last three released Swift versions, which are now: 5.9, 5.10, and 6.0.

Modifications:

• Bump the minimum tools version in the package manifest to 5.9.
• Remove some conditional compilation that we can assume going forward.
• Remove the Docker Compose file used for the 5.8 CI.
• Update the README indicating that the next minor (3.8.x) will be Swift 5.9+.

Result:

No longer actively maintaining support for Swift 5.8.

[simonjbeaumont]

@Lukasa when you have time.