Enable Swift 6 language mode in GRPCCore #2493

Job	Run time
Integration Tests on swiftlang/swift:nightly-jammy	4m 44s
Integration Tests on swiftlang/swift:nightly-6.0-jammy	7m 27s
Performance Tests on swiftlang/swift:nightly-jammy	3m 7s
Build and Test on swiftlang/swift:nightly-jammy	1m 55s
Integration Tests on swift:5.10.1-noble	5m 11s
Performance Tests on swiftlang/swift:nightly-6.0-jammy	3m 45s
License Header and Formatting Checks	3m 20s
Integration Tests on swift:5.9-jammy	4m 0s
Build and Test on swiftlang/swift:nightly-6.0-jammy	4m 0s
Performance Tests on swift:5.10.1-noble	31s
Integration Tests on swift:5.8-focal	3m 26s
Build and Test on swift:5.10.1-noble	3m 56s
Performance Tests on swift:5.9-jammy	33s
Build and Test on swift:5.9-jammy	3m 12s
Performance Tests on swift:5.8-focal	27s
Build and Test on swift:5.8-focal	3m 0s
	52m 34s

Provide feedback