exclude *.json, *.txt from license header check

- json files do not support comments in file header
- stdout.txt contains output of test
- fixes error in CI

.gitlab-ci.yml

@@ -73,6 +73,8 @@ check-license-header:
             ! grep \
                 --recursive \
                 --exclude-dir='.git' \
+                --exclude='*.json' \
+                --exclude='*.txt' \
                 --files-without-match "Apache License" \
                 .