Gate `ExperimentalCustomExecutors` usage behind Swift 6.4 compiler ch… #2509

Job	Run time
format	37s
prettier	13s
build-examples	22m 12s
Build for native target (macos-15, Xcode_16.4)	2m 55s
check-bridgejs-generated	2m 47s
Build and Test (ubuntu-24.04, https://download.swift.org/development/ubuntu2404/swift-DEVELOPMENT...	11m 40s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	13m 13s
Test BridgeJS against Swift versions (swift:6.1.2, 601.0.0)	2m 52s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	18m 11s
Build and Test (ubuntu-24.04, https://download.swift.org/swift-6.3-branch/ubuntu2404/swift-6.3-DE...	12m 58s
Test BridgeJS against Swift versions (swift:6.2, 602.0.0)	3m 11s
deploy-examples	10s
	1h 30m 59s

Provide feedback