Fix availability marker for Swift 5.9 compiler targeting host machine

[kateinoigakukun]

When building JavaScriptKit with Xcode, targeting host machine, the func enqueue(_: ExecutorJob) must be guarded by @available attribute explicitly. And due to the 5.9 compiler issue, it emit some migration warnings too consevatively. This commit suppress the warning by updating minimum available OS versions. Those versions should not be a problem when building for Wasm.

[fjtrujy]

Tested myself in my project, it is working

[github-actions]

Time Change: +313ms (3%)

Total Time: 9,511ms

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import with int	13ms	-1ms	✅
Serialization/JavaScript Number to Swift Int	329ms	+18ms (5%)	🔍
Serialization/JavaScript String to Swift String	3,708ms	+201ms (5%)	🔍

View Unchanged

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import	18ms	-0ms
Serialization/JavaScript function call from Swift	141ms	-3ms (2%)
Serialization/Swift Int to JavaScript with assignment	311ms	-1ms
Serialization/Swift Int to JavaScript with call	942ms	+22ms (2%)
Serialization/Swift String to JavaScript with assignment	391ms	+14ms (3%)
Serialization/Swift String to JavaScript with call	1,007ms	+37ms (3%)
Object heap/Increment and decrement RC	2,637ms	+28ms (1%)

View Baselines

Test name	Duration
Serialization/Call JavaScript function directly	3ms
Serialization/Assign JavaScript number directly	2ms
Serialization/Call with JavaScript number directly	3ms
Serialization/Write JavaScript string directly	2ms
Serialization/Call with JavaScript string directly	2ms