Bump minimum Swift version to 5.7

[rnro]

Motivation:

Now that Swift 5.9 is GM we should update the supported versions and remove 5.6

Modifications:

• Update Package.swift
• Remove #if swift(>=5.7) guards
• Delete the 5.6 docker compose file and make a 5.10 one
• Update docs

Result:

Remove support for Swift 5.6, add 5.10

[yim-lee]

CI updated. @swift-server-bot test this please