GitHub runner updates

[SWilson4]

This PR does the following:

• Adds the new (beta) GitHub runners for Windows 2025 and macOS 15.
• Updates PLATFORMS.md accordingly.
• Sets macOS runners to use gcc-14 (now supported by all runners) instead of gcc-13.
• Removes the GCC 13 patching logic from CI.

Closes #1804.

• 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.)
• 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.)