Drop support for Swift 5.10

[glbrntt]

• Remove 5.10 from CI and Makefile workarounds
• Remove 5.10 specific package manifests
• Fix deprecation warnings from 6.0
• Bump language mode to 6 and remove 'StrictConcurrency=Complete'

[thomasvl]

We can't do this until the oss-fuzz support for Swift is updated otherwise, that will start to fail to build.

[thomasvl]

ps - I opened an issue with them because it isn't clear how to update them: google/oss-fuzz#14033

[glbrntt]

Ah okay, no problem.

Moving to swift-tools-version: 6.0 is also what introduces the deprecation warnings in the SwiftPM plugin, so this PR depends on #1912 getting merged which in turn depends on dropping 5.10 support (or #if compiler checking a lot of code).

[thomasvl]

I should have said, the oss-fuzz side doens't prevent landing this, it's more a question of do you all (as owners of the repo) and to lose the fuzz coverage over there?

[glbrntt]

I should have said, the oss-fuzz side doens't prevent landing this, it's more a question of do you all (as owners of the repo) and to lose the fuzz coverage over there?

We should sort out oss-fuzz before landing this, it'd be a shame to lose fuzzing coverage. I'll look into it...