MNT: (deps): Bump flake8-simplify from 0.19.3 to 0.21.0 in /ci

Bumps [flake8-simplify](https://github.com/MartinThoma/flake8-simplify) from 0.19.3 to 0.21.0.
- [Release notes](https://github.com/MartinThoma/flake8-simplify/releases)
- [Changelog](https://github.com/MartinThoma/flake8-simplify/blob/main/CHANGELOG.md)
- [Commits](MartinThoma/flake8-simplify@0.19.3...0.21.0)

---
updated-dependencies:
- dependency-name: flake8-simplify
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

ci/linting_requirements.txt

@@ -11,7 +11,7 @@ flake8-mutable==1.2.0
 flake8-pep3101==1.3.0
 flake8-print==5.0.0
 flake8-quotes==3.3.1
-flake8-simplify==0.19.3
+flake8-simplify==0.21.0
 pep8-naming==0.13.2
 
 flake8-rst-docstrings==0.2.7