[MM-40952] migrations: Add if statements for some queries (#19359)

* migrations: add if statements for some queries

* add better checks to upgrade posts v6 migration

* use "is null" instead of "= null"

* fix down script for users v6

* reflect review comments

db/migrations/postgres/000017_create_roles.up.sql

@@ -14,18 +14,21 @@ CREATE TABLE IF NOT EXISTS roles (
 ALTER TABLE roles ADD COLUMN IF NOT EXISTS builtin boolean;
 
 DO $$
-	<< migrate_roles >>
+	<< migrate_if_version_below_500 >>
+DECLARE
+	current_db_version VARCHAR(100) := '';
 BEGIN
-	IF((
-		SELECT
-			value
-		FROM systems
+	SELECT
+		value INTO current_db_version
+	FROM
+		systems
 	WHERE
-		Name = 'Version') = '4.10.0') THEN
-            UPDATE Roles SET SchemeManaged = false
+		name = 'Version';
+	IF (string_to_array(current_db_version, '.') < string_to_array('5.0.0', '.')) THEN
+		 UPDATE Roles SET SchemeManaged = false
             WHERE Name NOT IN ('system_user', 'system_admin', 'team_user', 'team_admin', 'channel_user', 'channel_admin');
 	END IF;
-END migrate_roles
+END migrate_if_version_below_500
 $$;
 
 ALTER TABLE roles ALTER COLUMN permissions TYPE TEXT;