build: use c++20 for all C++ files, not just those that use the seastar flags
build: use c++20 for all C++ files, not just those that use the seastar flags

A few source files (like those generated by antlr) don't build with seastar,
and so don't inherit all of its flags. They then use the compiler default dialect,
not C++20. With gcc that's just fine, since gcc supports concepts in earlier dialects,
but clang requires C++20.

Fix by forcing --std=gnu++20 for all files (same as what Seastar chooses).

Closes #7392

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla