Consider dropping support for .NET Framework 4.7.2

[KrzysFR]

We currently still support both .NET Standard 2.0 and .NET Framework 4.7.2 for "legacy" platforms, in addition to .NET Core 3.1 / .NET 5.0

Time as passed since the early days of .NET Standard 2.0 where support was a bit flaky, so I think that only keeping .NET Standard 2.0 around for some time is sufficient, and any new development should be done on the latest .NET version (6.0 at the moment). Any application targeting .NET Core 3.x should be migrated to .NET 6.0 since it is the new long term support version of .NET.

Having support for both .NET Standard 2.0 and .NET 5.0 seems like a good middle ground.

Actions proposed:

• Remove the net472 target from libraries (and only keep netstandard2_0)
• Maybe keep net472 for the tools? (they would use the .NET Standard 2.0 libraries)? We could consider only supporting .NET 5.0+ for these as well...