Add a warning box about the cgroupv1 escape hatch

[twz123]

Description

KEP-5573 states that cgroup v1 support will be removed eventually. Make that clear in the docs by adding a warning box with the respective quote. Also reword the paragraph above a little bit and hard-wrap it.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[k0s-bot]

Successfully created backport PR for release-1.36:

• [Backport release-1.36] Add a warning box about the cgroupv1 escape hatch #7823