mingw-w64: Update to 12.0.0

[Gcenx]

UCRT is now the default CRT runtime instead of MSVCRT, so need to rev-bump *-w64-mingw32-gcc Ports at the same time.

See https://www.mingw-w64.org/changelog/#v1200-2024-05-29

Description

Type(s)

Tested on

macOS 14.5 23F79 x86_64
Xcode 15.4 15F31d

Verification

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@mojca for port i686-w64-mingw32-gcc, mingw-w64, x86_64-w64-mingw32-gcc.

[Gcenx]

Also added myself as co-maintainer of mingw-w64