restrict Windows platform support documentation [skip ci]

[baentsch]

oqs-provider has been testing cygwin support also for liboqs. But this now fails for two reasons:

• OpenSSL 3.4 triggers a crash
• Activating the pytest toolchain in cygwin is not seamlessly possible.

This PR therefore restricts stated Windows support to MSVC until a formal decision is taken to enhance Windows support beyond this (& adding CI in this project).

• [no] Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• [no] Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged.)

[SWilson4]

Fine with me. Thanks for taking the time to formalize this, @baentsch.

[baentsch]

Fine with me. Thanks for taking the time to formalize this, @baentsch.

Not so much meant as formalization but reminder that OQS has a problem with cygwin (that may be worth while fixing as and if there'd be interest).