spread.yaml: stop testing on Ubuntu 21.04

With the imminent release of Ubuntu 22.04 and 21.04 being out-of-support
window, we should start removing older releases from the test matrix.

The extra CPU time spent on this is not bringing any important alignment
with what's available in Oniro at this time.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>