Support for GCC 7 and 8 can continue on an as-needed basis. The
goal is not to explicitly break support for such compilers. Chances
are they will continue to compile Catch2 for some time into the
future. Rather, it no longer seems a prudent use of resources to
continuously test with these compilers.
The base images for GitHub Actions are updated weekly, but
sometimes that is not enough to be able to install the packages
we require. The recommended fix for this is to always run
`apt-get update` before `apt-get install`.
* Fixed problem with older compilers by explicitly installing the
right compiler version.
* Split apart simple builds that can be well described by a build
matrix, and builds that need special CMake defines or similar
special snowflake handling.
* Ported some extras + examples builds from TravisCI to GitHub
Actions.
