Switch to pkgconf (MSYS2, MinGW-w64)

Switch to pkgconf (MSYS2, MinGW-w64)

The MSYS2 project has switched from pkg-config to pkgconf as default.
For details, see: https://github.com/msys2/MSYS2-packages/pull/2197
Both packages work when building RRDtool for Windows using MSYS2.
- Switch to pkgconf instead of pkg-config
- Update the following files:
  .github/workflows/ci-workflow.yml
  win32/README-MinGW-w64
- This avoids the following warning in the Windows CI-build:
  warning: removing 'mingw-w64-x86_64-pkg-config' from target list
  because it conflicts with 'mingw-w64-x86_64-pkgconf'