[msgpack] update version to 6.0.0 and switch to the C++ branch

[FrankXie05]

Fixes #30389 #28117 #30615

Note: In the latest version, msgpack divides the C and C++ libraries into different branches.
The current port msgpack is supported as a C++library.

• Changes comply with the maintainer guide
• SHA512s are updated for each updated download
• The "supports" clause reflects platforms that may be fixed by this new version
• Any fixed CI baseline entries are removed from that file.
• Any patches that are no longer applied are deleted from the port's directory.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is added to each modified port's versions file.

[kotori2]

I think boost is no longer required in the latest version, and it builds prefectly without it. Could you remove it from vcpkg.json?

[FrankXie05]

@kotori2 Officially, they only rely on boost.
https://github.com/msgpack/msgpack-c/tree/cpp_master#:~:text=msgpack%2Dc%20requires%20boost%20library.%20C%2B%2B%20version%20of%20msgpack%2Dc%20itself%20is%20a%20header%2Donly%20library%20and%20depends%20only%20on%20boost%20headers.%20Tests%20depend%20on%20boost%20unit%20test%20framework%20and%20are%20linked%20with%20it%2C%20so%20if%20you%20want%20to%20build%20them%2C%20you%20need%20to%20have%20this%20dependency%20installed.

[kotori2]

@kotori2 Officially, they only rely on boost. https://github.com/msgpack/msgpack-c/tree/cpp_master#:~:text=msgpack%2Dc%20requires%20boost%20library.%20C%2B%2B%20version%20of%20msgpack%2Dc%20itself%20is%20a%20header%2Donly%20library%20and%20depends%20only%20on%20boost%20headers.%20Tests%20depend%20on%20boost%20unit%20test%20framework%20and%20are%20linked%20with%20it%2C%20so%20if%20you%20want%20to%20build%20them%2C%20you%20need%20to%20have%20this%20dependency%20installed.

Sorry but it seems there was some issue with Github code search.
I downloaded their source code and it does depend on boost.
But the Boost they mentioned is different from what it is in vcpkg repository. Boost contains hundreds of sub-packages, and the boost package in vcpkg will simply install all of them, which is not the best option here.
I created a test project and it seems that msgpack only requires these boost packages:

  "dependencies": [
    "boost-numeric-conversion",
    "boost-fusion",
    "boost-optional",
    "boost-variant"
  ],

Could you please change them in the current port?

[FrankXie05]

@kotori2 Sure, this reduces the build time.
Thank you for your testing and investigation.