Update LICENSE.txt (mattermost#10512)

The `config/` folder is licensed under Apache 2.0.

Previously, this folder contained data files for default.json and timezones.json. However, in the past month, we have repurposed the config folder, which will autogenerate the config file in an upcoming release.

To account for the restructuring of the config folder, `config/` is no longer considered to be licensed with Apache 2.0. In the near term, we'll keep `config/default.json` licensed under Apache 2.0 for compatibility with previous releases, and then later remove it from the list once `default.json` is replaced with the auto-generation of the config file.

LICENSE.txt

@@ -11,7 +11,7 @@ You may be licensed to use source code to create compiled versions not produced
 1. Under the Free Software Foundation’s GNU AGPL v.3.0, subject to the exceptions outlined in this policy; or
 2. Under a commercial license available from Mattermost, Inc. by contacting commercial@mattermost.com
 
-You are licensed to use the source code in Admin Tools and Configuration Files (templates/, config/, model/,
+You are licensed to use the source code in Admin Tools and Configuration Files (templates/, config/default.json, model/,
 plugin/ and all subdirectories thereof) under the Apache License v2.0.
 
 We promise that we will not enforce the copyleft provisions in AGPL v3.0 against you if your application (a) does not