Add 5.3+trunk

[jmid]

Following the fork of the 5.2 branch on ocaml/ocaml and ocaml/opam-repository#24986
this PR

• adjusts 5.2.0+trunk+win package to point to the 5.2 branch
• adds a 5.3.0+trunk+win package pointing to the trunk branch

Here's a diff for easy comparison:

packages/ocaml-variants$ diff ocaml-variants.5.2.0+trunk+win/opam ocaml-variants.5.3.0+trunk+win/opam 
3c3
< synopsis: "Latest OCaml 5.2 development version, configured for Windows"
---
> synopsis: "OCaml development version, configured for Windows"
8c8
< dev-repo: "git+https://github.com/ocaml/ocaml.git#5.2"
---
> dev-repo: "git+https://github.com/ocaml/ocaml.git#trunk"
10c10
<   "ocaml" {= "5.2.0" & post}
---
>   "ocaml" {= "5.3.0" & post}
92c92
<   src: "https://github.com/ocaml/ocaml/archive/5.2.tar.gz"
---
>   src: "https://github.com/ocaml/ocaml/archive/trunk.tar.gz"

[shym]

Thanks!