Update extension docs to mention GNU GPLv3 is a valid license (#40933)

Merge after: 

- zed-industries/extensions#3641

Release Notes:

- N/A

Author: JosephTLyons

docs/src/extensions/developing-extensions.md

@@ -113,8 +113,9 @@ git submodule update
 
 As of October 1st, 2025, extension repositories must include one of the following licenses:
 
-- [MIT](https://opensource.org/license/mit)
 - [Apache 2.0](https://www.apache.org/licenses/LICENSE-2.0)
+- [GNU GPLv3](https://www.gnu.org/licenses/gpl-3.0.en.html)
+- [MIT](https://opensource.org/license/mit)
 
 This allows us to distribute the resulting binary produced from your extension code to our users.
 Without a valid license, the pull request to add or update your extension in the following steps will fail CI.