This change reverts some changes in the unit tests that were hacks
required in previous kit builds. Some of the issues have been fixed in
more recent versions.

We are now also setting `/app/player/useFixedTimeStepping=False` in
Isaac Lab as Isaac Sim used to do this by default, but is no longer
applying this setting in 5.0.

We are also dropping official support for Ubuntu 20.04 and adding
support for Ubuntu 24.04.

<!-- As you go through the list, delete the ones that are not
applicable. -->

- Bug fix (non-breaking change which fixes an issue)
- New feature (non-breaking change which adds functionality)
- Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- This change requires a documentation update

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

<!--
As you go through the checklist above, you can mark something as done by
putting an x character in it

For example,
- [x] I have done this task
- [ ] I have not done this task
-->