CI: Test on 4.02, 4.04 and remove macos

4.02 (the lower supported version) and 4.04 (lower bound for many tests)
  are no longer tested in ocaml-ci.

The macos build is redundant with ocaml-ci and slows down the CI
significantly.

.github/workflows/build.yml

@@ -10,14 +10,15 @@ jobs:
     strategy:
       matrix:
         os:
-          - macos-latest
           - ubuntu-latest
-          # - windows-latest Windows doesn't work yet
         ocaml-compiler:
           # Don't include every versions. OCaml-CI already covers that
+          - 4.02.x
+          - 4.04.x
           - 4.14.x
         include:
           - os: ubuntu-latest # Enable coverage only on a single build
+            ocaml-compiler: 4.14.x
             send-coverage: true
       fail-fast: false