Generally developers compile with recent compilers, so bumping the ci configs to a recent OS should be uncontroversial. Older OSes (especially with compiler sanitizers) need workarounds that can be dropped by running on a more recent OS.
This pull changes the asan sanitizer and the experimental multiprocess build to use focal.
Also, it runs the no_wallet config on xenial to test against python 3.5, according to doc/dependencies.md
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Finally, all configs that mimic gitian (win and mac) will stay at bionic.