* Updates are no longer available for CentOS Stream 8 since the end of May 2024
* Updates will cease being available for CentOS 7 at the end of June 2024
_And_:
* Remove Fedora 38
* Fix `openSUSE Leap 15` Docker build's default GCC no longer compiles `Mapnik` latest
* C++17 is now required, so a newer GCC will be installed
* Fix for coverage capturing with LCOV 2.1
* Seems to currently only exist on macOS
* Previously it was not actually sending the header
* Now runs in serial, regardless of parallel specification
* Now runs only once, rather than for each map layer
_Also_:
* Removed no-longer used `storage_mutex`
* Was only used for Apache HTTP Server < v2.4
* Report coverage for almost all jobs
* Some jobs fail and are omitted
* Add more tests to `gen_tile_test.cpp`
* Also lightly cleaned up formatting
* Clean up cmake/* files
* So they don't report an empty version
* Remove commented `check_*` lines from `CMakeLists.txt` file
* These were left over from the initial CMake support implementation
For linting `CMakeLists.txt` & `*.cmake` files (`cmakelint`) as well as various other formats such as `HTML`, `MarkDown` & `YAML` (`prettier`.)
### Also:
* Upgrade `actions/checkout` from `v3` to `v4`
* Output coverage summary to GitHub Job step summary
* Minor build docs cleanup for `CentOS` & `FreeBSD`
* Copy the Ubuntu 20.04 docs to Debian 12
* Update debian docs to refer to proper location of example map
* update debian docs to match what mapnik installs
* Update debian docs to match the installed URL
* Link to debian docs in README
