Add contrib/upgrade.sh (#18286)

Adds an upgrade script that automates upgrading installations on Linux from binary releases, so people don't need to reinvent the wheel. Hopefully this leads to less questions about how to upgrade, and consequently less Gitea instances running unmaintained versions in the wild.

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>

docs/content/doc/upgrade/from-gitea.en-us.md

@@ -76,6 +76,8 @@ a snapshot for the Gitea data volume and related object storage is more convenie
 * Replace the installed Gitea binary with the downloaded one. 
 * Start the Gitea instance.
 
+A script automating these steps for a deployment on Linux can be found at [`contrib/upgrade.sh` in Gitea's source tree](https://github.com/go-gitea/gitea/blob/main/contrib/upgrade.sh).
+
 ## Take care about customized templates
 
 Gitea's template structure and variables may change between releases, if you are using customized templates,