Migrate to Swift 6.0+ and Swift 6 language mode

[czechboy0]

Motivation

We were still using Swift 5 language mode.

Modifications

• enabled Swift 6 language mode
• updated CI to 6.0+
• added most of our standard upcoming feature flags and updated code to get zero warnings

Left out one flag: NonisolatedNonsendingByDefault - will look into that separately, as it might break our API if we're not careful.

Result

Modernized code.

Test Plan

Locally passes without warnings on macOS and Linux 6.2. The rest to be tested on CI.

[czechboy0]

Merging over the red API breakage checker, as it doesn't seem to respect @preconcurrency:

5 breaking changes detected in OpenAPIRuntime:
  💔 API breakage: func AsyncSequence.asDecodedJSONLines(of:decoder:) has generic signature change from <Self, Event where Self : Swift.Sendable, Self : _Concurrency.AsyncSequence, Event : Swift.Decodable, Self.Element == Swift.ArraySlice<Swift.UInt8>> to <Self, Event where Self : Swift.Sendable, Self : _Concurrency.AsyncSequence, Event : Swift.Decodable, Event : Swift.SendableMetatype, Self.Element == Swift.ArraySlice<Swift.UInt8>>
  💔 API breakage: func AsyncSequence.asDecodedJSONLines(of:decoder:) is now with @preconcurrency
  💔 API breakage: func AsyncSequence.asDecodedJSONSequence(of:decoder:) has generic signature change from <Self, Event where Self : Swift.Sendable, Self : _Concurrency.AsyncSequence, Event : Swift.Decodable, Self.Element == Swift.ArraySlice<Swift.UInt8>> to <Self, Event where Self : Swift.Sendable, Self : _Concurrency.AsyncSequence, Event : Swift.Decodable, Event : Swift.SendableMetatype, Self.Element == Swift.ArraySlice<Swift.UInt8>>
  💔 API breakage: func AsyncSequence.asDecodedJSONSequence(of:decoder:) is now with @preconcurrency
  💔 API breakage: func AsyncSequence.asDecodedServerSentEventsWithJSONData(of:decoder:while:) is now with @preconcurrency