temporarily enable latest stable 15

Author: acao

.github/workflows/pr-graphql-compat-check.yml

@@ -24,6 +24,8 @@ jobs:
     strategy:
       matrix:
         release:
+          # try last stable 15 release again
+          - '^15'
           # test against the latest 16.x version, which might be newer than what we have
           - '^16'
           # test against the oldest version we support