Raise minimum supported Swift version from 5.4 to 5.5

[glbrntt]

Motivation:

SwiftNIO periodically drops support for older Swift versions. Now that 5.7 has been released, 5.4 will be dropped.

Modifications:

• Remove 5.4 specific Package.swift and docker-compose
• Update the 5.7 docker-compose to use the released 5.7 and move from focal (2004) to jammy (2204)
• Fix the tools version in Package@swift-5.5.swift
• Update docs

Results:

Minimum Swift version is 5.5

[glbrntt]

~We should ship a release with https://github.com/apple/swift-nio-ssl/pull/400 in so any folks who can't upgrade beyond 5.4 have warning free builds.~

400 was a test-only fix.

[yim-lee]

CI (including performance jobs) updated. @swift-server-bot test this please