tool_getpass: restore UWP `getpass_r()`, fixup CI builds, fix UWP `-Wnull-dereference`

tool_getpass: restore UWP `getpass_r()`, fixup CI builds, fix UWP `-Wnull-dereference`

- GHA/windows: switch mingw-w64 UWP CI job to use UCRT.
  `msvcr120_app` was missing `getch()` for example.
  Follow-up to f988842d85a06d7ad03764433b6dfee9febf0118 #15637
  This job tests compiling for UWP correctly, but the the resulting
  `curl.exe` still doesn't look like a correct UWP app, now exiting
  on startup with: `curl: error initializing curl library`.

- tool_getpass: restore `getch()` for UWP builds.
  Follow-up to f988842d85a06d7ad03764433b6dfee9febf0118 #15637

- schannel: silence `-Werror=null-dereference` warning in mingw-w64 UWP:
  ```
  lib/vtls/schannel_verify.c: In function 'Curl_verify_host':
  lib/vtls/schannel_verify.c:558:33: error: null pointer dereference [-Werror=null-dereference]
    558 |     for(i = 0; i < alt_name_info->cAltEntry; ++i) {
        |                    ~~~~~~~~~~~~~^~~~~~~~~~~
  lib/vtls/schannel_verify.c:559:50: error: null pointer dereference [-Werror=null-dereference]
    559 |       PCERT_ALT_NAME_ENTRY entry = &alt_name_info->rgAltEntry[i];
        |                                     ~~~~~~~~~~~~~^~~~~~~~~~~~
  ```
  Ref: https://github.com/curl/curl/actions/runs/12022656065/job/33515255397?pr=15638#step:19:27
  Follow-up to 9640a8ef6f0db21da2ee7864423aa28cc3cecce2 #15421

- GHA/windows: fix `find` command in MSVC job step.
  Follow-up to 5f9411f953f35ca84645b5a6824fb9fef20887ed #15380

- GHA/windows: drop unnecessary `windowsappcompat` lib from mingw-w64
  UWP job. Also drop related MSYS2 package.

- GHA/windows: cmake 3.31.0 still invokes `windres` with wrong options
  with mingw-w64 UPW. Update curl version in comment accordingly.

- GHA/windows: tidy up mingw-w64 UWP spec logic, limit it to gcc.

- GHA/windows: update comments on `curl.exe` UWP startup errors.

Closes #15638