Build failure with older ocaml versions (< 4.13)

[snapdgn]

While I was working on fixing the build workflow, I noticed that upstream sail uses, two version 4.08.1 & 5.0.0 of ocaml to test the build.

We currently use String.starts_with & String.ends_with in our json backend(json.ml), which were introduced in ocaml version 4.13.0. Consequently, attempting to build our backend with versions prior to 4.13.0 results in failure.
The same applies to find_map. There may be other instances as well.

Given that upstream continues to use version 4.08.1 for testing builds, it seems sensible to maintain backward compatibility.
The only question is how far back we should provide support?