os.yml: drop support for 4.12, check also 5.2

And later we could switch to 5.3, to ensure compatibility with recent compilers

Author: mseri

.github/workflows/os.yml

@@ -15,10 +15,10 @@ jobs:
           - ubuntu-latest
           # - windows-latest
         ocaml-compiler:
+          - "5.2"
           - "5.1"
           - "4.14"
           - "4.12"
-          - "4.10"
 
     runs-on: ${{ matrix.os }}