Update prometheus-client requirement from <0.8,>=0.7 to >=0.7,<0.22

Updates the requirements on [prometheus-client](https://github.com/prometheus/client_python) to permit the latest version.
- [Release notes](https://github.com/prometheus/client_python/releases)
- [Commits](prometheus/client_python@v0.7.0...v0.21.1)

---
updated-dependencies:
- dependency-name: prometheus-client
  dependency-type: direct:production
...

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

requirements.txt

@@ -28,7 +28,7 @@ pycups==2.0.4  # https://pypi.python.org/pypi/pycups
 PyPDF2>=1.26,<3.1  # https://github.com/mstamy2/PyPDF2/blob/master/CHANGELOG
 
 # Only for cmsPrometheusExporter
-prometheus-client>=0.7,<0.8  # https://pypi.org/project/prometheus-client
+prometheus-client>=0.7,<0.22  # https://pypi.org/project/prometheus-client
 
 # Only for cmsTelegramBot
 python-telegram-bot>=21.9,<21.10 # https://pypi.org/project/python-telegram-bot