* MM-36743: adds last_root_post_at in channels table
Channel recency for CRT users should not count replies,
this commit solves that issue by adding a new column to the channels
table: LastRootPostAt.
With that new info CRT users can have recent channels to work as
expected.
* Adds the 'LastRootPostAt' to the SQL scripts
* Fixes tests
* Adds LastRootPostAt migration to v6.1
* Fixes index
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>