ci: don't test against node 13 (#16)

Author: G-Rath

.github/workflows/checks.yaml

@@ -33,7 +33,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        node-version: [10.x, 12.x, 13.x, 14.x]
+        node-version: [10.x, 12.x, 14.x]
         eslint-version: [6, 7]
     runs-on: ubuntu-latest