[ci][default] allow setting max_cxx_standard and add C++20 pipeline

What this MR does / why does DuMux need it:

Adds a pipeline with C++20 and dune master, and pushes the corresponding pipeline with C++17 from MR to the scheduled pipelines.

Notes for the reviewer

Fixes #1169 (closed)

Depends on !3227 (merged) to be merged first

• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.