Remove all code that solely existed to support Swift 5.8 #1335

Job	Run time
api-breakage	3m 38s
linux-integration-and-dependencies (postgres:17)	8m 56s
linux-unit (swift:5.9-jammy)	2m 38s
macos-all (postgresql@16, scram-sha-256, ~15)	2m 11s
linux-unit (swift:5.10-noble)	3m 27s
linux-integration-and-dependencies (postgres:15)	8m 47s
linux-unit (swift:6.0-noble)	3m 18s
linux-integration-and-dependencies (postgres:12)	9m 0s
linux-unit (swiftlang/swift:nightly-main-jammy)	3m 50s
	45m 45s

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