libstdc++: Remove dg-options "-std=gnu++20" from std::format tests

The testsuite will automatically select C++20 for these tests now, and
removing the hardcoded -std option allows them to be tested for C++23
and C++26 as well.

libstdc++-v3/ChangeLog:

	* testsuite/std/format/arguments/args.cc: Remove dg-options.
	* testsuite/std/format/arguments/lwg3810.cc: Likewise.
	* testsuite/std/format/error.cc: Likewise.
	* testsuite/std/format/format_string.cc: Likewise.
	* testsuite/std/format/formatter/concept.cc: Likewise.
	* testsuite/std/format/formatter/ext_float.cc: Likewise.
	* testsuite/std/format/formatter/requirements.cc: Likewise.
	* testsuite/std/format/functions/107871.cc: Likewise.
	* testsuite/std/format/functions/format.cc: Likewise.
	* testsuite/std/format/functions/format_to.cc: Likewise.
	* testsuite/std/format/functions/format_to_n.cc: Likewise.
	* testsuite/std/format/functions/size.cc: Likewise.
	* testsuite/std/format/functions/vformat_to.cc: Likewise.
	* testsuite/std/format/parse_ctx.cc: Likewise.
	* testsuite/std/format/string.cc: Likewise.
	* testsuite/std/format/string_neg.cc: Likewise.
	* testsuite/std/format/functions/format_c++23.cc: Removed.