cmake: fix `pkg-config`-based detection in `FindGSS.cmake`

cmake: fix `pkg-config`-based detection in `FindGSS.cmake`

Before this patch `pkg-config`-based detection was ignored, and used
solely as a path hint for native detection.

- fix `pkg_search_module()` result prefix to match what code expects:
  `_GSS` (was: `_GSS_PKG`). Update variable that were in sync with old
  prefix.

- update the pkg-config codepath to use `_GSS_MODULE_NAME` to detect
  GSS flavour. This requires CMake 3.16.
  Otherwise fall back to the old method. (The old method doesn't seem to
  work anymore (?) as of CMake 3.30.1. Documented
  `<prefix>_<modulename>_VERSION` variable is defined, but empty.)

- update the pkg-config codepath to use `_GSS_VERSION` set by CMake.
  Resort to the old code when this variable is empty. (The old code
  doesn't seem to work anymore (?) as of CMake 3.30.1)

- fix pkg-config codepath to set the documented result variables.

- align native detection variable names with those generated by
  `pkg_search_module()` in the pkg-config codepath.

- GHA/macos: enable GSS Heimdal in a cmake job.
  Uses the native detection.

- GHA/linux: enable GSS Heimdal in cmake and autotools jobs.
  CMake uses `pkg-config`-based detection.

- suppress test 2077 and 2078 results on Linux + Heimdal.
  ```
  FAIL-IGNORED 2077: 'curl --fail --negotiate to unauthenticated service fails' HTTP, HTTP GET, GSS-API
  FAIL-IGNORED 2078: 'curl --negotiate should not send empty POST request only' HTTP, HTTP GET, GSS-API
  ```
  Failing with valgrind errors in both autotools and cmake builds:
  https://github.com/curl/curl/actions/runs/10282222581/job/28453472068?pr=14430#step:38:3638
  https://github.com/curl/curl/actions/runs/10282222581/job/28453473398?pr=14430#step:38:7831

Closes #14430