Bump matplotlib from 3.7.2 to 3.9.2 in /.github/dependabot

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 3.7.2 to 3.9.2.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.7.2...v3.9.2)

---
updated-dependencies:
- dependency-name: matplotlib
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/dependabot/constraints.txt

@@ -2,7 +2,7 @@ cartopy==0.23.0
 cftime==1.6.2
 click==8.1.7
 pandas<2.2.3
-matplotlib==3.7.2
+matplotlib==3.9.2
 netCDF4==1.6.4
 numpy==1.25.2
 scipy==1.14.1