Add 24.04 base support for Flask and Django extensions. The current `stable` release of Charmcraft supports the `ubuntu@24.04` base for Flask and Django, which I validated by running through the tutorials manually. However, the Spread tests fail with the error `Extension ... does not support base(s): ubuntu@24.04`, indicating that some change in the `edge` release of Charmcraft has dropped support for 24.04 in Flask and Django. I'm not sure what change prompted this behavior, but this PR explicitly defines the base support in the Flask and Django frameworks, which resolves the failures in the Spread tests. This change was originally part of https://github.com/canonical/charmcraft/pull/2732 which we thought might have been fixed in a different PR ([comment thread](https://github.com/canonical/charmcraft/pull/2732#discussion_r3443178623)). However, the Spread tests failed after reverting the change in the PR (which was merged before the Spread tests finished running). * Example of failed job: https://github.com/canonical/charmcraft/actions/runs/28617084008/job/84867369161?pr=2732 * Example of failure: `Extension 'django-framework' does not support base(s): ubuntu@24.04` --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/charmcraft/blob/main/CONTRIBUTING.md). - [x] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [ ] I've successfully run `make lint && make test`. - [ ] I've added or updated any relevant documentation. - [ ] In documents I changed, I [added a meta description](https://canonical-starflow.readthedocs-hosted.com/how-to/add-a-page-meta-description/) if one was missing. - [ ] I've updated the relevant release notes.