Removal of c++17 support in v01-03

[tmadlener]

We will be removing the support for building podio with c++17 in v01-03 of podio! (#698). Starting from v01-02 we will make building with c++20 the default and introduce a cmake warning when building with c++17 (#700).

The most important impact for users will be that a ROOT build with c++20 enabled is required.

In case you are still using c++17 for building podio and have requirements that make moving to c++20 impossible please let us know (e.g. by a comment to this issue).

This is technically a breaking change since we remove a "feature". However, we have decided against bumping the major version of podio for this for now, as the API and the generated code remains unchanged.