Test suite: require Bullseye or newer

With Bullseye released mid-2021, it should be reasonable to start requiring Bullseye or newer to run the test suite soonish.

Let's migrate once we can assume Bullseye on all machines that run the test suite:

• developers (update doc and announce the change)
• RMs
  • check with boyska
• Jenkins: sysadmin#17739 (closed)

Once this is done, we can update some code:

• test suite: migrate from deprecated luks_open and luks_close to cryptsetup_{open,close}, to remove some warnings