Upgrading to C++20

[lpugin]

No major changes beside:

• Updating make files
• Moving the LedgerLine definition up (before `Staff) because of one compiler complaining about it
• Ajust the CI (remove unsupported OS / compiler combinations and add new ones)

If upgrading to C++20 is a problem, please speak up. It is not absolutely necessary to do this now, but at the same time there seems to be no reason no to do it.

[craigsapp]

It should not be a problem for me.

On this page: https://developer.apple.com/xcode/cpp

The features supported by Xcode are listed.

One thing of note is that the referenced page says XCode 15 is required for many of the new C++20 features:

And there is a fair amount of C++20 features that Xcode does not support yet (so don't use those).

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Here are the C++20 features that GCC supports:

https://gcc.gnu.org/projects/cxx-status.html#cxx20

Of note:

Important: Because the ISO C++20 standard is very recent, GCC's support is experimental.

and a note about compiler flags in GCC:

C++20 features are available since GCC 8. To enable C++20 support, add the command-line parameter -std=c++20 (use -std=c++2a in GCC 9 and earlier) to your g++ command line. Or, to enable GNU extensions in addition to C++20 features, add -std=gnu++20.

But that should not be a problem since -std=c++20 should work in GCC 10 and higher.

GCC 11 is required for all implemented features (on my up-to-date linux I am using GCC 13) and a on slightly older version of linux I am using GCC 12.

[lpugin]

Thanks for checking this. This is the status of the building tests on GH
https://github.com/rism-digital/verovio/actions/runs/7583048214

They all look fine. There is however one problem on Windows https://ci.appveyor.com/project/LaurentPugin/verovio-r1t6l/builds/49024120