when targeting at least C++20

These methods were added to C++20, so they are not available in
earlier language standards. Users requiring compatibility prior to C++20
should use absl::StartsWith() and absl::EndsWith() from
//absl/strings/match.h.

Most users are not affected by this change. By default when targeting at least
C++20 absl::string_view will be an alias for std::string_view. Only users
that have modified //absl/base/options.h will see this change.

PiperOrigin-RevId: 575238435
Change-Id: I7b03fde02c987b30b88c794640c2a616851997d1