Drop support for Swift 5.5

[simonjbeaumont]

Motivation:

Now Swift 5.8 is released, we are dropping support for 5.5, in line with the usual support window.

Modifications:

• Remove Package@swift-5.5.swift.
• Remove #if swift(>=5.6) compiler guards.
• Update documentation and README.

Result:

No functional change, but Swift 5.5 is no longer supported.

[Lukasa]

@swift-server-bot add to allowlist

[simonjbeaumont]

@yim-lee Please can we remove the 5.5 CI for this project?

[yim-lee]

Please can we remove the 5.5 CI for this project?

@simonjbeaumont Done