CI: Get rid of buildjet-2vcpu-ubuntu-1804 GHA runners

[mjameswh]

Description

Our CI workflows are still using buildjet-2vcpu-ubuntu-1804 runners to build the Test Server binaries, so that we maintain glibc compatibility with various, older Linux distributions.

GitHub dropped support for Ubuntu 18.04 about 2 years ago, but until now, it was still possible for external builders (like Buildjet's) to continue using that distribution. However, the latest release of GitHub's Runner Agent brings in new dependencies that are incompatible with Ubuntu 18.04, forcing immediate update of all Runners.

It is therefore urgent that we update our CI workflows to use a more recent runner, as we might be incapable of making a new Java SDK release until this is fixed.