- Update 1.21.4-changelog.rst.
- Update 1.21.4-notes.rst.

- Create the 1.21.4-changelog.rst file.
- Update the 1.21.4-notes.rst file.

- Create 1.21.2-changelog.rst
- Update 1.21.2-notes.rst

- Update the release 1.21.1-notes.rst
- Add release notes to index
- Create the 1.21.1-changelog.rst
- Delete towncrier notes fragment

Backport of #19314.

Bump scipy-mathjax submodule. This is actually a no-op (npm is used only if you upgrade the mathjax stored in scipy-mathjax to a newer version manually, last done in 2017), but silences some code quality scanners.

The package.json, rebuild.sh could also be removed from the shipped documentation as they do nothing there, but not sure if that's worth the hassle.