Bump eslint-config-etherpad from 3.0.13 to 3.0.21

Bumps [eslint-config-etherpad](https://github.com/ether/eslint-config-etherpad) from 3.0.13 to 3.0.21.
- [Commits](ether/eslint-config-etherpad@v3.0.13...v3.0.21)

---
updated-dependencies:
- dependency-name: eslint-config-etherpad
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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