CI: Test with OCaml 5.3 in Github CI

Some versions are removed because they are already tested in ocaml-ci.
Opam-repository must be specified for 5.3 as it's not updated by
default.

.github/workflows/build.yml

@@ -18,16 +18,19 @@ jobs:
           - "4.08"
           - "4.10"
           - "4.12"
-          - "4.14"
-          - "5.0"
           - "5.2"
+          - "5.3"
         include:
           - os: ubuntu-latest
             ocaml-compiler: "5.2"
             # We don't need to compute coverage for more than one build
             send-coverage: true
             # Mdx tests Mdx tests
             run-mdx: true
+          - os: ubuntu-latest
+            ocaml-compiler: "5.3"
+            opam-repositories:
+              default: https://github.com/ocaml/opam-repository.git
           - os: macos-latest
             ocaml-compiler: "5.2"
           # - os: windows-latest