Update Messaging interface for Swift 6

[morganchen12]

Makes Messaging mostly @MainActor. Also migrates references of Swift 5 and Xcode 15 to Swift 6 and Xcode 16.

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[google-oss-bot]

	1 Warning
⚠️	Did you forget to add a changelog entry? (Add #no-changelog to the PR description to silence this warning.)

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[ncooke3]

Should we label the callback with @MainActor?

[ncooke3]

@MainActor (Notification) -> Void

[morganchen12]

Doing this causes a compiler error, that's why it has to be checked dynamically via the precondition later on.