build-unit-test-docker: update ubuntu to hirsute

The latest Ubuntu "Hirsute Hippo" was released on 2021-04-22.
Switch our CI builds to use it.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If41dfd2247f0513bc297c4789cb051f7f69cbec4
1 file changed