Fixing issue with Swift 6.0 and DateEncodingStrategy

[leogdion]

It looks like Swift 6.0 removed the DateEncodingStrategy.formatted(_) case. This patches that and adds CI runs for newer versions of Ubuntu and Swift.

[mattpolzin]

The failing CI for the bionic and amazonlinux containers for Swift 5.8 and 5.9 at least are known issues — we can just remove those specific entries from the test matrix.
[EDIT] apparently that’s the same issue even for 5.10 and 6.0 for the amazonlinux containers. Remove those too. Not worth testing against them if it’s going to be a pain.

[leogdion]

I added an XCTSkip since we can run those tests in Linux

[mattpolzin]

I'm not totally clear on why Swift 6 on Linux stopped being able to support some of the test cases that earlier Swift versions supported on Linux but ideally the tests would only be skipped for the specific Linux test runs where they are expected to fail so we don't introduce regressions for support of Linux with Swift versions prior to 6 by accident.

[jcbriones]

I was about to do the same changes but thanks for opening this up. I do wonder too why that got removed in 6.0. 🤔