Patch warnings for conformances of external types

[marcosgriselli]

Add the source compatible patch for https://github.com/swiftlang/swift-evolution/blob/main/proposals/0364-retroactive-conformance-warning.md to avoid warnings on Swift 6+ and still be able to compile on previous versions.

#trivial

[rxswiftcommunity[bot]]

It looks like code was changed without adding anything to the Changelog.

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