Expose `WebWorkerTaskExecutor` even if compiling with toolchain < 6.1 #1218

Job	Run time
Build for embedded target (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-...	47s
Build for native target (macos-14, Xcode_15.2)	20s
Build for native target (macos-15, Xcode_16)	14s
Build and Test (macos-12, wasm-5.9.1-RELEASE, Node, Xcode_13.4.1.app)	53s
Build and Test (macos-13, wasm-5.9.1-RELEASE, Node, Xcode_14.3.app)	54s
Build and Test (macos-14, wasm-5.9.1-RELEASE, Node, Xcode_15.2.app)	57s
Build and Test (ubuntu-22.04, wasm-5.9.1-RELEASE, Node)	1m 4s
Build and Test (ubuntu-22.04, wasm-5.10.0-RELEASE, Node)	1m 5s
Build and Test (ubuntu-20.04, wasm-5.8.0-RELEASE, Node)	1m 6s
Build and Test (ubuntu-20.04, wasm-5.10.0-RELEASE, Node)	1m 7s
Build and Test (ubuntu-20.04, wasm-5.8.0-RELEASE, MicroWASI)	1m 8s
Build and Test (ubuntu-20.04, wasm-5.9.1-RELEASE, MicroWASI)	1m 9s
Build and Test (ubuntu-20.04, wasm-5.10.0-RELEASE, MicroWASI)	1m 8s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	51s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	1m 7s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	1m 6s
	14m 56s

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