AVRO-3995 [C++] Require C++17 to compile Avro

[Gerrit0]

What is the purpose of the change

AVRO-3995 Update the CMake configuration to provide a useful error message when attempting to compile with an unsupported C++ standard version, and remove code which was #ifdef'd to be used only with C++ targets less than C++17.

As discussed in #2431, Avro has effectively required C++17 for nearly a year now, but it wasn't obvious to consumers.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Documentation

• Does this pull request introduce a new feature? No
• If yes, how is the feature documented? N/A

[Fokko]

@Gerrit0 Can you rebase now #2931 is in?

[Gerrit0]

@Fokko done!

[martin-g]

Thank you, @Gerrit0 !