Set RUN_MODE prod by default (#13765)

I think it's a bad default to have "dev" as the default run mode which
enables debugging and now also disables HTTP caching. It's better to
just default to a value suitable for general deployments.

Co-authored-by: techknowlogick <[email protected]>

doc/advanced/config-cheat-sheet.en-us.md

@@ -36,9 +36,7 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `APP_NAME`: **Gitea: Git with a cup of tea**: Application name, used in the page title.
 - `RUN_USER`: **git**: The user Gitea will run as. This should be a dedicated system
    (non-user) account. Setting this incorrectly will cause Gitea to not start.
-- `RUN_MODE`: **dev**: For performance and other purposes, change this to `prod` when
-   deployed to a production environment. The installation process will set this to `prod`
-   automatically. \[prod, dev, test\]
+- `RUN_MODE`: **prod**: Application run mode, affects performance and debugging. Either "dev", "prod" or "test".
 
 ## Repository (`repository`)