In addition, we delete the hard-coded Fenix specific `version` and we
use `abbrev`, instead.

We need to make sure we just set MOZ_BUILD_DATE and adapt the format
where it is actually needed. See: fenix#40059 for the Fenix part that
makes use of that to fix #40084.

We bump Fenix to 81.1.1 and android-components to 57.0.6 while GeckoView
is staying on 81.0b9 to save us another round of rebasing.

It turns out that declaring a `beta_version` and including that in the
version number for a branch name might not have been the best idea given
that not all our branches are based on mozilla-beta. So, we drop that
and add the beta version directly when needed.

We sort now in a way that makes the output reproducible on different
machines and exclude .module files, too, as we do not need them.

Versions bump and Changelog update

Additionally, we clean up our projects removing Linux workarounds not
needed under Jessie anymore.

CentOS 6 will be EOL later this year (Nov 2020). We are removing support
for it beginning with 10.5 which will get released around mid-2021.

Apparently we can't just replace the torbrowser-android-* targets with
android-* ones when including the Rust compilers for application-services.
Thus, we revert that part of the patch for #40070. See: #40094 for more
details.

Versions bump and Changelog update

This inclues the build time improvements that landed in #33289 and
rbm#40005.

We are in shape now for having Fenix-based nightly builds.

https://bugzilla.mozilla.org/show_bug.cgi?id=1612423 does that for us
now when using mingw-w64.