Add \c mattermost step to PostgreSQL preparation grants (#8920)

* Add \c mattermost step to PostgreSQL preparation grants

The ALTER SCHEMA public and GRANT USAGE, CREATE ON SCHEMA public
commands operate on the current database's public schema, so users
must connect to the mattermost database before running them.
Without \c mattermost, these grants are applied to the wrong
database's public schema.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Add comment explaining the \c mattermost step

Clarifies that the connection switch is what makes the subsequent
ALTER SCHEMA and GRANT commands target the mattermost database's
public schema.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Switch code-block to postgresql to handle \c cleanly

The generic sql lexer flags \c as an unknown token (red error box
in rendered output). The postgresql lexer recognizes psql
meta-commands like \c as builtins, so the block renders cleanly
while still highlighting the SQL statements.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Use text lexer to avoid Pygments error token on \c

Both the sql and postgresql Pygments lexers flag backslash-prefixed
psql meta-commands as error tokens (rendered as a red box). Switch
to the text lexer so the block renders cleanly. Loses SQL syntax
highlighting on four lines, but keeps the commands as a single
copy-pasteable unit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ewwollesen

source/deployment-guide/server/preparations.rst

@@ -59,9 +59,11 @@ PostgreSQL v14+ is required for Mattermost server installations. :doc:`MySQL dat
 
    e. If using PostgreSQL v15.x or later, additional grants are required:
 
-      .. code-block:: sql
+      .. code-block:: text
 
          ALTER DATABASE mattermost OWNER TO mmuser;
+         -- Connect to the mattermost database so the schema grants below apply to the right schema
+         \c mattermost
          ALTER SCHEMA public OWNER TO mmuser;
          GRANT USAGE, CREATE ON SCHEMA public TO mmuser;