Deprecating configure-style in opam files

[emillon]

The "configure-style" field is parsed but ignored:

https://github.com/ocaml/opam/blob/4d9db690420e36d995e6a0afe18660bcb45bea49/src/format/opamFile.ml#L2780

Since it's been deprecated and ignored for a long time it looks like it should be removed in 2.1 or 2.2.

[dra27]

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