Update Windows bootstrap dependencies

[MisterDA]

I've noticed that some of the dependencies could be updated. The updates could help when bootstrapping or building opam under Windows.

• FlexDLL 0.44 (update from 0.43) brings fixes for linking with long filenames, linking under the multicore runtime, and fixes for cross-compilation. See the release notes and release announcement. FlexDLL 0.44 has already been published to the opam-repository and is used by all up-to-date switches on Windows.
• msvs-detect 0.7.0 (update from 0.5.0) brings support for the latest Visual Studio, and various fixes. See release notes.
• mingw-w64 13.0.0 (update from 12.0.0, CI only) has various improvements, but this shouldn't affect us too much (changelog).

I'm a bit dissatisfied to "dump" this PR on the maintainers this late in the release process, without being able to check if the CI follows through, but I don't have much more time available.

[kit-ty-kate]

I'm a bit dissatisfied to "dump" this PR on the maintainers this late in the release process

I think it's ok as i don't think this should be part of the upcoming opam 2.4 which was branched already and whose release candidate has already been released. This PR will do fine for opam 2.5 though i think