Deprecating "syntax" in opam files

[emillon]

The "syntax" field is supported but does not seem that anything in the ecosystem is using it:

https://github.com/ocaml/opam/blob/4d9db690420e36d995e6a0afe18660bcb45bea49/src/format/opamFile.mli#L463-L464

Might be worth deprecating it in 2.1 or 2.2.

[dra27]

See https://github.com/ocaml/opam/issues/4519#issuecomment-868445795