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 guards for swift <5.7
• Add deprecation annotations to LegacyTracer API
• 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]

@swift-server-bot add to allowlist

[yim-lee]

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

[ktoso]

@swift-server-bot test this please

[ktoso]

Ok all jobs failing here due to warnings as errors etc.

I'll look into addressing this soon -- unless you're still up for it @rnro ?

[ktoso]

I'll handle this -- the fallout is the deprecation warnings since we have warnings as errors...

[ktoso]

Done, thank you! I'll call a 1.1.0 with this