Removed old instructions about Ubuntu 18.04 as there's no good reason
to use such an old OS to build those containers, and we're not testing
it so chances are that the containers may not build/work properly there.
It saves a lot of trouble to just use latest Fedora (or possibly RHEL/CentOS)
to build those containers.
We now provide pre-built toolchains for Linux with old glibc and the required
Godot dependencies, which can be used to build portable Godot binaries without
having to be on an extremely old Linux distro.
So we can retire the old Ubuntu 14.04 containers and instead do our portable
builds directly on Fedora 34, like for other platforms.
This commit also makes use of `godot-mono-builds` to build Mono and the BCL
for Linux, instead of doing it manually. The same will be done for Windows and
macOS in the next commit.
