MM-62782: Bumping minimum Postgres version to 13 (#30039)

As discussed in https://docs.mattermost.com/install/prepare-mattermost-database.html#minimum-supported-version-policy

https://mattermost.atlassian.net/browse/MM-62782

```release-note
Minimum postgres version has been bumped to 13.
```

Co-authored-by: Mattermost Build <build@mattermost.com>

e2e-tests/.ci/server.generate.sh

@@ -78,7 +78,7 @@ $(for service in $ENABLED_DOCKER_SERVICES; do
 $(if mme2e_is_token_in_list "postgres" "$ENABLED_DOCKER_SERVICES"; then
     echo '
   postgres:
-    image: mattermostdevelopment/mirrored-postgres:12
+    image: mattermostdevelopment/mirrored-postgres:13
     restart: "no"
     network_mode: host
     networks: !reset []