chore: Re-enable opam matrix

Author: spotandjake

.github/workflows/opam.yml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        ocaml-compiler: [5.3.0]
+        ocaml-compiler: [4.13.1, 4.14.1, 5.3.0]
 
     steps:
       - name: Setup node.js