modules;json: inconsistent keys: `is-standard-library` vs `is-std-library`

[mathstuf]

@mordante In libc++.modules.json, there are two spellings for "is this a standard library module": is-standard-library and is-std-library. This should be resolved before 18.0.0 is released. Introduced in #75741.

Pinging release managers to get this marked as a blocker for 18.0: @llvm/llvm-release-managers