MM-36743: adds last_root_post_at in channels table (#18366)

* 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>

scripts/mattermost-postgresql-6.0.sql

@@ -105,6 +105,7 @@ CREATE TABLE public.channels (
     header character varying(1024),
     purpose character varying(250),
     lastpostat bigint,
+    lastrootpostat bigint,
     totalmsgcount bigint,
     extraupdateat bigint,
     creatorid character varying(26),