[6.1] Update SwiftStdlib 6.1 availability macro

[slavapestov]

6.1 cherry-pick of #80596.

• Description: Fill in the real version numbers now that Swift 6.1 has shipped.

• Reviewed by: @hborla

[slavapestov]

@swift-ci Please test

[slavapestov]

/home/build-user/swift/include/swift/AST/RuntimeVersions.def:158:3: error: use of undeclared identifier 'PLATFORM_TEST_visionOS'
  PLATFORM(visionOS,(2, 4, 0))
  ^
/home/build-user/swift/lib/AST/Availability.cpp:706:7: note: expanded from macro 'PLATFORM'
  if (IS_PLATFORM(P))                                                          \
      ^
/home/build-user/swift/lib/AST/Availability.cpp:708:24: note: expanded from macro 'IS_PLATFORM'
#define IS_PLATFORM(P) PLATFORM_TEST_##P
                       ^
<scratch space>:48:1: note: expanded from here
PLATFORM_TEST_visionOS
^
1 error generated.

Oops.

[slavapestov]

@swift-ci Please test