* libass: update contact list
One contact is retired; another is currently inactive.
* libass: don't build HarfBuzz tests
They won't be used, so this just wastes resources.
* libass: upgrade to Ubuntu 20.04
The build failures on 20.04 were due to explicit listing
of static library dependencies, which have changed on 20.04:
png12 was replaced by png16, fontconfig was missing -luuid,
and png needed -lz to follow it, not precede.
Replace the explicit -l list by $(pkg-config --static --libs).
However, -lm must be filtered out as it causes linker errors:
> undefined reference to `_dl_x86_cpu_features'
While we are at it, Ubuntu 20.04 comes with new-enough FriBidi,
so we can stop building it.
It also has new-enough HarfBuzz, but it depends on libgraphite2,
which lacks a static library in the system package, and dynamically
linking against a system library does not work in OSS-Fuzz
as the shared object will not exist in the run-time environment.
Reorder the dependencies alphabetically for consistency.