pg_table/conversation: Remove access_role and make epoch_timestamp a timestamptz

The access_role column is not used anymore, so we can just dervie the value of
access_roles_v2 from it and persist that.

The timestamptz type maps to UTCTime, which we use everywhere, so this is more convenient.

Author: akshaymankar

libs/wire-subsystems/postgres-migrations/20250908142515-create-conversation-tables.sql

@@ -1,15 +1,14 @@
 CREATE TABLE conversation (
   id uuid PRIMARY KEY,
   access integer[],
-  access_role integer,
   access_roles_v2 integer[],
   cells_state integer,
   channel_add_permission integer,
   cipher_suite integer,
   creator uuid,
   deleted boolean,
   epoch bigint,
-  epoch_timestamp timestamp,
+  epoch_timestamp timestamptz,
   group_conv_type integer,
   group_id bytea,
   message_timer bigint,