libstdc++: Simplify std::pair constraints using concepts

libstdc++: Simplify std::pair constraints using concepts

This re-implements the constraints on the std::pair constructors and
assignment operators in C++20 mode, to use concepts.

The non-standard constructors deprecated for PR 99957 are no longer
supported in C++20 mode, which requires some minor testsuite changes.
Otherwise all tests pass in C++20 mode.

libstdc++-v3/ChangeLog:

	* include/bits/stl_pair.h (pair) [__cplusplus > 202002]: Add
	new definitions for constructors and assignment operators using
	concepts for constraints.
	* testsuite/20_util/pair/cons/99957.cc: Disable for C++20 and
	later.
	* testsuite/20_util/pair/cons/explicit_construct.cc: Adjust
	expected error messages to also match C++20 errors.