[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/channels/utils/license_test.go

@@ -112,7 +112,8 @@ func TestGetLicenseFileFromDisk(t *testing.T) {
 		f, err := os.CreateTemp("", "TestGetLicenseFileFromDisk")
 		require.NoError(t, err)
 		defer os.Remove(f.Name())
-		os.WriteFile(f.Name(), []byte("not a license"), 0777)
+		err = os.WriteFile(f.Name(), []byte("not a license"), 0777)
+		require.NoError(t, err)
 
 		fileBytes := GetLicenseFileFromDisk(f.Name())
 		require.NotEmpty(t, fileBytes, "should have read the file")