Add support for Swift 5.9, drop support for previous versions of Swift #100

Job	Run time
generate-and-publish-docs / spm-context	50s
generate-and-publish-docs / basic-context	5s
generate-and-publish-docs / generate-docs (RouteDocs)	5m 10s
generate-and-publish-docs / publish-docs	7s
generate-and-publish-docs / cleanup	2s
	6m 14s

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