Use StrictConcurrency=complete in Package.swift and add Swift 6 language mode CI

[simonjbeaumont]

Motivation

We're trying to lock in strict concurrency across the package ecosystem and standardise on how that's tested.

Modifications

• Add StrictConcurrency=complete feature flag to all targets in the package manifest
• Remove now superfluous strict concurrency compiler flag in CI runs
• Add a new CI pipeline that tests the package with Swift 6 language mode.

Result

• Package configuration consistent with others adopting strict concurrency.
• Swift 6 language mode pipeline established to catch regressions.