enable StrictConcurrency build setting

[weissi]

Set

swiftSettings: [
    .enableExperimentalFeature("StrictConcurrency")
]

so we know where there's work.

[weissi]

This is currently incompatible with warnings-as-errors, so we need some CI tweaks to handle it.

Ah yeah, should I just tweak the CI?

[Lukasa]

That can work, though @FranzBusch is in the process of cutting over to a new CI scheme so we may want to hold off and tackle this slightly later.

It's also not clear that AHC can easily do this without us sorting out NIO first.