[CMake] Update dependency finders.

1. Fixes CMake using any found HarfBuzz version lower than the minimum
required. This is based on HALX99's merge request at
https://gitlab.freedesktop.org/freetype/freetype/-/merge_requests/31
2. Update FindHarfBuzz.cmake from
https://github.com/WebKit/WebKit/blob/1ce32454/Source/cmake/FindHarfBuzz.cmake
and guard post-CMake-3.1 features to keep the minimum version unchanged
3. Update FindBrotliDec.cmake to stop the warnings, based on what
https://github.com/google/woff2/blob/a0d0ed7d/cmake/FindBrotliDec.cmake
is doing

* CMakeLists.txt, builds/cmake/FindHarfBuzz.cmake: Implement 1 and 2.
* builds/cmake/FindBrotliDec.cmake: Implement 3.
4 files changed