CI: Extend OCaml version coverage

Cover OCaml versions between 4.02 and 4.14 as ocaml-ci stopped covering
them.

4.02 is the lower supported version and 4.04 is the lower bound for
generator tests. Other versions are included for completeness.
To limit the number of CI jobs, only impair versions are covered.

The macos build is removed because it's redundant with ocaml-ci and
slows down the CI significantly.

Author: Julow

.github/workflows/build.yml

@@ -10,14 +10,18 @@ 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
+          - 4.06.x
+          - 4.08.x
+          - 4.10.x
+          - 4.12.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