OrderedSet: Fix sendable conformance on old swifts

OrderedSet has lost the condition swift(>=5.5) around its Sendable conformance, and that predictably causes issues on Swifts that predate that.

[X] I've read the Contribution Guidelines
[X] My contributions are licensed under the Swift license.
[X] I've followed the coding style of the rest of the project.
[ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
[ ] I've added benchmarks covering new functionality (if appropriate).
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[ ] I've updated the documentation if necessary.

apple / swift-collections