This removes all CI builds from travis-ci, due to their recent changes
in policy and harsh limitations on builds. With build times over 2
hours, it was a significant hindrance to development.
Instead of Travis CI, the CI is now split on Sourcehut and GitHub. Since
Sourcehut only supports Linux/BSD, all builds on those operating systems
are executed there. The GitHub Actions CI is used to build for
Windows/macOS, which are not available on Sourcehut.
Since asset deployment for releases requires builds on all platforms,
this is also done on GitHub actions. Though the new `upload_asset.sh`
script makes sure that migration in the future is fairly simple and we
do not tie ourselves to the overly complicated GitHub Actions ecosystem.
To make the release process a bit smoother and prevent a freeze of the
master process while review candidates are out, this will put the master
in a perpetual development state.
This should make it clear to everyone that the official source for
releases is always the tagged branch and make it possible to release new
versions completely independently.
Since versions are bumped after each release, this makes it so the
release branches do not have to get merged back into the master branch
to show the correct development version.
