Use a .ci/cicd-requirements.txt file to handle CI/CD only dependencies

[misl6]

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

Over the past few years it happened multiple times that our CI/CD pipeline started to fail due to unpinned dependencies.

These changes are meant to move all the CI/CD specific dependencies into a separate requirements file, to avoid polluting the CI/CD scripts with multiple pip install statements.

This change also allows us to keep these dependencies versions monitored via renovatebot.

Cons:

• Now all the requirements specified in cicd-requirements.txt get installed even if something may not be needed in a specific run. We can avoid that by using cached dependencies, however, we're talking about small-sized requirements.