NonisolatedNonsendingByDefault and other concurrency fixes

[czechboy0]

Motivation

• Fixes Consider moving to Swift 6.2 as minimum version #17
• Fixes combineLatestOneOrMore is not correctly bubbling up errors thrown by providers #23
• Fixes Enable NonisolatedNonsendingByDefault #32
• Fixes ConfigUpdatesAsyncSequence - should it be Sendable or passed as sending? #43
• Fixes Add a CI pipeline that builds with the explicit sendable setting #44

These 5 fixes are intertwined and were difficult to split out, so here's one PR that fixes them all.

Modifications

• The PR enables NonisolatedNonsendingByDefault, which caused an error in the existing combineLatest implementation, which we wanted to replace anyway.
• So we vendor in the code from Swift Async Algoritms (the PR is open here: Add a combineLatestMany algorithm swift-async-algorithms#360), the plan is to remove this copy when it lands in async algos.
• Updating the combineLatest implementation to the correct one requires the async sequences to be Sendable, which in turn requires Swift 6.2 for us to be able to spell any (AsyncSequence & Sendable) correctly, it doesn't work on 6.1.
• We also enabled the explicit sendable warning in CI, helping us ensure all our public API is explicitly annotated as Sendable or not.

Result

Addressed 5 important issues that impact the API and concurrency.

Test Plan

Brought over tests for the new combineLatest implementation, added more unit tests for failure cases when using the ConfigReader with multiple providers.

[czechboy0]

API breakage is red because adding nonisolated(nonsending) is detected as a breaking change, no other breakages.