libjuju version hardcoded to 2.9.9

[Ashley James]

Currently in requirements.txt [1], the libjuju version is hardcoded to 2.9.9. We could replace this with:

```
juju ~= 2.9
```

which would allow the snap to install the latest 2.9.x compatible version [2] (2.9.42 currently). This would allow the snap to receive the latest libjuju improvements and fixes.

[1]: https://git.launchpad.net/juju-backup-all/tree/requirements.txt
[2]: https://peps.python.org/pep-0440/#compatible-release

[Mert Kirpici (mertkirpici)]

Since charm-juju-backup-all installs juju-backup-all through python package not snap, during dependency resolution process of the charm build, charm also ends up getting libjuju version 2.9.9. Marking this bug affects the charm project as well.

[Eric Chen (eric-chen)]

The teams is working on `compatible with juju 3.1`. After that, this issue should be fixed. Therefore, I remove `good-first-bug` tag.