🍒 Cherry pick StdlibDeploymentTarget to 6.2

[al45tair]

We need this to avoid assertion failures or crashes from the Concurrency runtime.

rdar://155214488

[al45tair]

@swift-ci Please test

[al45tair]

Explanation: The existing availability set-up had a problem wherein it was difficult or impossible to use newly added code to reimplement older code because the availability annotations are applied even within the defining module. To fix this, I'd added a new StdlibDeploymentTarget availability macro, which solves the problem by setting the macro to match the build system, unless we place it into strict mode.
Risk: Medium. This is quite a large changeset and it changes the build system. On the other hand, it has been in use on main for some time now.
Original PR: #81440 #81940
Reviewed by: @ktoso, @stephentyrone, @edymtt
Resolves: rdar://155214488 (rdar://148275746, rdar://152498657)
Tests: The test suite should exercise these changes already.