Add pipeline in C++20 mode

We want to support C++20 and use it in one of the upcoming releases. So we should start testing with C++20.

changed milestone to %3.6

added Dev / CI Dumux Day Feature Request labels

assigned to @DennisGlaeser

@RoWin

mentioned in merge request !3172 (merged)

mentioned in commit 9fec51a9

closed with merge request !3172 (merged)