Merge bitcoin/bitcoin#30228: build: no-longer allow GCC-10 in C++20 check

232928b build: no-longer allow GCC-10 in C++20 check (fanquake)

Pull request description:

  Reverts part of fa67f09, now that we require a minimum of GCC 11.

  See also:
  bitcoin/bitcoin#28349 (comment).

ACKs for top commit:
  maflcko:
    utACK 232928b
  theuni:
    utACK 232928b

Tree-SHA512: 10e0adac2dd6e455aaf97ebfe73c7586430349fc27ac435bc6c0d99a4934a380398d14467aacd9cedf371345da291366b3ab2c3be7db5d97e21ad6212b2c7890

Author: fanquake

build-aux/m4/ax_cxx_compile_stdcxx.m4

@@ -983,7 +983,7 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_20], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus < 201709L  // Temporary patch on top of upstream to allow g++-10
+#elif __cplusplus < 202002L
 
 #error "This is not a C++20 compiler"