[MM-61082] Fix errcheck issues in license_test.go (#30617)

* [MM-28754] Fix errcheck issues in license_test.go

- Properly handle error returned from os.WriteFile
- Remove license_test.go from errcheck exceptions list in .golangci.yml

Fixes https://github.com/mattermost/mattermost/issues/28754

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Remove empty line

---------

Co-authored-by: Claude <noreply@anthropic.com>

server/.golangci.yml

@@ -160,7 +160,6 @@ issues:
         channels/store/storetest/team_store.go|\
         channels/store/storetest/thread_store.go|\
         channels/store/storetest/user_store.go|\
-        channels/utils/license_test.go|\
         channels/web/oauth_test.go|\
         channels/web/saml.go|\
         channels/web/web_test.go|\