nixos/mattermost: remove fallback charset for MySQL

This was causing issues on newer versions of MariaDB (breaking
NixOS tests) like:

```
Error 1064 (42000): You have an error in your SQL syntax;
check the manual that corresponds to your MariaDB server version
for the right syntax to use near '%2Cutf8' at line 1
```

Since this is simply a fallback character set and all supported versions
of MariaDB support utf8mb4, delete the fallback.

This change should be fully compatible with existing deployments.

(cherry picked from commit 6cc8a8cdb5)

nixos/modules/services/web-apps/mattermost.nix

@@ -668,7 +668,7 @@ in
               }
             else if cfg.database.driver == "mysql" then
               {
-                charset = "utf8mb4,utf8";
+                charset = "utf8mb4";
                 writeTimeout = "60s";
                 readTimeout = "60s";
               }
@@ -682,7 +682,7 @@ in
               }
             else if config.mattermost.database.driver == "mysql" then
               {
-                charset = "utf8mb4,utf8";
+                charset = "utf8mb4";
                 writeTimeout = "60s";
                 readTimeout = "60s";
               }