Update our CI jobs to cover the 3.6 stable branch

Update our CI jobs to cover the 3.6 stable branch

The coveralls, prov-compat and provider-compatibiity CI jobs test each
of the stable branches.  We need to add 3.6 to the list in each of those
tests

Fixes openssl/project#1424

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/28519)