ostc-docs-builder: revert back to focal

Having backported sphinx from groovy, we can fall back to focal. This
is more in line with our installation instructions which recommend focal
anyway, and more in sync with what most developers are using daily.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>
2 jobs for !13 with tweak/docs-backported in 1 minute and 18 seconds (queued for 8 seconds)
latest detached
Status Job ID Name Coverage
  Compliance
passed #9285
dco

00:00:11

 
  Build
passed #9286
build-docs-ostc-builder

00:01:06