Bump selenium from 4.9.1 to 4.21.0 in /requirements

Bumps [selenium](https://github.com/SeleniumHQ/Selenium) from 4.9.1 to 4.21.0.
- [Release notes](https://github.com/SeleniumHQ/Selenium/releases)
- [Commits](SeleniumHQ/selenium@selenium-4.9.1...selenium-4.21.0)

---
updated-dependencies:
- dependency-name: selenium
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements/dev.in

@@ -20,7 +20,7 @@ pytest-rerunfailures==14.0
 pytest-selenium==4.0.1
 responses==0.25.0
 ruff==0.4.2
-selenium==4.9.1  # Pinned to 4.9.1 until https://github.com/pytest-dev/pytest-selenium/issues/315 is resolved
+selenium==4.21.0  # Pinned to 4.9.1 until https://github.com/pytest-dev/pytest-selenium/issues/315 is resolved
 translate-toolkit==3.12.2
 wagtail-factories==4.1.0
 # Related to moz-l10n-lint, used in CI