Returned Github Actions support for py27 and py36.

Returned Github Actions support for py27 and py36.
GitHub recently upgraded the `ubuntu-latest` label from `ubuntu-20.04` to `ubuntu-22.04`.
The `ubuntu-22.04` image removed support for py27 and py36.

To return support, the affected jobs have been duplicated to `-legacy` versions.
The -legacy versions of jobs run py27 and py36 on a pinned `ubuntu-20.04` os.
The existing jobs continue to run py37+ on ubuntu-latest, as that platform may continue to benefit from patch releases on the python versions.

Change-Id: I0f063723cb993fab89bc64c89df6dfcaf4dbe5a5