Enable Swift 6 language mode in GRPCCore (#2046) #2528

Job	Run time
License Header and Formatting Checks	3m 1s
Integration Tests on swiftlang/swift:nightly-jammy	4m 36s
Build and Test on swiftlang/swift:nightly-jammy	2m 2s
Integration Tests on swiftlang/swift:nightly-6.0-jammy	9m 53s
Performance Tests on swiftlang/swift:nightly-jammy	3m 1s
Build and Test on swiftlang/swift:nightly-6.0-jammy	6m 51s
Integration Tests on swift:5.10.1-noble	5m 8s
Performance Tests on swiftlang/swift:nightly-6.0-jammy	3m 22s
Build and Test on swift:5.10.1-noble	3m 59s
Integration Tests on swift:5.9-jammy	4m 11s
Performance Tests on swift:5.10.1-noble	27s
Build and Test on swift:5.9-jammy	3m 10s
Performance Tests on swift:5.9-jammy	31s
Integration Tests on swift:5.8-focal	3m 18s
Build and Test on swift:5.8-focal	2m 52s
Performance Tests on swift:5.8-focal	24s
	56m 46s

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