Use MicroCeph tag for docs build

[roosterfish]

Currently we pin a specific commit on MicroCeph's main branch for the combined docs build. We should rather use the upstream https://github.com/canonical/microceph/releases/tag/v19.2.0%2Bsquid tag instead as it gets updated and will itself resolve to MicroCeph's standalone stable docs.

[UtkarshBhatthere]

@roosterfish I always wanted to make the move from the naming v19.2.0+squid to v19+squid since it follows the same convention as desired by RTD but allows from removing the sub-versioning that doesn't make much sense for microceph docs (as it is independent of the ceph package version). Kindly mark this one as blocked, I will update here once i change it.