gh-35177: Matplotlib 3.7.0 support
### 📚 Description
Matplotlib 3.7.0 introduce some changes to the labelling of objects in
subplots (mainly "subplot" is gone). This PR fixes the affected
doctests.
There are also some deprecations warnings that can be dealt with by
using objects already available in matplotlib 3.6.
Finally one user warning has gone away.
### 📝 Checklist
- [*] I have made sure that the title is self-explanatory and the
description concisely explains the PR.
- [ ] I have linked an issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/35177
Reported by: François Bissey
Reviewer(s): François Bissey, Gonzalo Tornaría
