Fix and improve the opam file

[kit-ty-kate]

This PR:

• Uses {dev} instead of {pinned} for the dune subst command (see Opam file generation: Use {dev} instead of {pinned} when calling dune subst ocaml/dune#3647)
• Removes mentions of unsupported versions of OCaml
• Restrict the ppxlib dependency to < 0.14.0. Currently fails with this error with ppxlib.0.14.0:

#=== ERROR while compiling ppx_deriving.dev ===================================#
# context              2.0.7 | linux/x86_64 | ocaml-base-compiler.4.10.0 | pinned(git://github.com/ocaml-ppx/ppx_deriving.git#e6cbddb8)
# path                 ~/.opam/4.10/.opam-switch/build/ppx_deriving.dev
# command              ~/.opam/4.10/bin/dune build -p ppx_deriving -j 71
# exit-code            1
# env-file             ~/.opam/log/ppx_deriving-146-2b4347.env
# output-file          ~/.opam/log/ppx_deriving-146-2b4347.out
### output ###
# Error: This expression has type Ppxlib.core_type list
# [...]
#     ocamlopt src/api/.ppx_deriving_api.objs/native/ppx_deriving.{cmx,o} (exit 2)
# (cd _build/default && /home/opam/.opam/4.10/bin/ocamlopt.opt -w -9 -g -I src/api/.ppx_deriving_api.objs/byte -I src/api/.ppx_deriving_api.objs/native -I /home/opam/.opam/4.10/lib/base -I /home/opam/.opam/4.10/lib/base/base_internalhash_types -I /home/opam/.opam/4.10/lib/base/caml -I /home/opam/.opam/4.10/lib/base/shadow_stdlib -I /home/opam/.opam/4.10/lib/ocaml-compiler-libs/common -I /home/o[...]
# File "ppx_deriving.cppo.ml", line 412, characters 68-69:
# Error: This expression has type Ppxlib.core_type list
#        but an expression was expected of type
#          Migrate_parsetree__Migrate_parsetree_versions.OCaml_408.Ast.Parsetree.core_type
#          list
#        Type Ppxlib.core_type = Parsetree.core_type
#        is not compatible with type
#          Migrate_parsetree__Migrate_parsetree_versions.OCaml_408.Ast.Parsetree.core_type
#            = Migrate_parsetree.Ast_408.Parsetree.core_type 

[gasche]

The improvements look good, thanks! But the CI fails on (ocaml >= 4.07):

- File "src/dune", line 8, characters 29-44:
- 8 |  (libraries ppx_deriving_api findlib.dynload compiler-libs.common)
-                                  ^^^^^^^^^^^^^^^
- Error: Library "findlib.dynload" not found.

I haven't had a time to look at it (I'm still on holidays), but this looks like a missing ocamlfind library that was previously hidden by ocamlfind being a recursive dependency. If you agree, could you add back the ocamlfind dependency?

[kit-ty-kate]

ah indeed it must have been hidden by the ppxlib dependency. I'll put ocamlfind instead

[gasche]

Thanks! This looks good now. (There is a transient CI failure for 4.06 but it looks network-related and unrelated to the change.)