c++: Change c++2b and gnu++2b to c++23 and gnu++23 in C++ diagnostics

This is something we should have done when -std=c++23 was made the
primary option and -std=c++2b turned into undocumented alias.

2025-01-16  Jakub Jelinek  <jakub@redhat.com>

gcc/cp/
	* parser.cc (cp_parser_lambda_declarator_opt,
	cp_parser_statement, cp_parser_selection_statement,
	cp_parser_jump_statement): Use -std=c++23 and -std=gnu++23
	in diagnostics rather than -std=c++2b and -std=gnu++2b.
	* semantics.cc (finish_compound_literal): Likewise.
	* typeck2.cc (build_functional_cast_1): Likewise.
	* decl.cc (start_decl): Likewise.
	* constexpr.cc (ensure_literal_type_for_constexpr_object,
	potential_constant_expression_1): Likewise.
gcc/c-family/
	* c-lex.cc (interpret_float): Use -std=c++23 and -std=gnu++23
	in diagnostics rather than -std=c++2b and -std=gnu++2b.