Addition and adjustments in connection with 5.3 branching

[jmid]

Following the upstream 5.3 branching and the updates in the opam-repo, this PR adds

• a ocaml-variants.5.4.0+trunk+win package and
• adjusts the existing ocaml-variants.5.3.0+trunk+win accordingly

Until an alpha/beta/rc is tagged, we keep the ocaml-variants.5.3.0+trunk+win name to match the opam-repo.

Here's a diff of the two resulting opam-files to highlight the small differences:

custom-opam-repository/packages$ diff ocaml-variants/ocaml-variants.5.3.0+trunk+win/opam ocaml-variants/ocaml-variants.5.4.0+trunk+win/opam
3c3
< synopsis: "OCaml 5.3 development version, configured for Windows"
---
> synopsis: "OCaml development version, configured for Windows"
8c8
< dev-repo: "git+https://github.com/ocaml/ocaml.git#5.3"
---
> dev-repo: "git+https://github.com/ocaml/ocaml.git#trunk"
10c10
<   "ocaml" {= "5.3.0" & post}
---
>   "ocaml" {= "5.4.0" & post}
92c92
<   src: "https://github.com/ocaml/ocaml/archive/5.3.tar.gz"
---
>   src: "https://github.com/ocaml/ocaml/archive/trunk.tar.gz"

[shym]

Thanks!