tests: Don't run py test_rlimit test on newer kernels

tests: Don't run py test_rlimit test on newer kernels

Since commit d5299b67dd59 ("bpf: Memcg-based memory accounting for bpf
maps"), memory locked by bpf maps is no longer counted against rlimit.

Ubuntu 20.04's 5.11 kernel has this commit, so we should skip this test
there. When we add future distros to github actions it may be necessary
to modify the version check here.