[gcc r12-6704] c++: Use -std=c++20 in testsuite default std list

[Jason Merrill]

https://gcc.gnu.org/g:fdd6d85bd75df9a97659220373b5c1e8b3019e26

commit r12-6704-gfdd6d85bd75df9a97659220373b5c1e8b3019e26
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Jan 18 17:43:03 2022 -0500

    c++: Use -std=c++20 in testsuite default std list
    
    C++20 has been official for a while now.
    
    gcc/testsuite/ChangeLog:
    
            * lib/g++-dg.exp: Change 2a to 20.

Diff:
---
 gcc/testsuite/lib/g++-dg.exp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gcc/testsuite/lib/g++-dg.exp b/gcc/testsuite/lib/g++-dg.exp
index ea7ec9e2635..a1bc37074b5 100644
--- a/gcc/testsuite/lib/g++-dg.exp
+++ b/gcc/testsuite/lib/g++-dg.exp
@@ -52,7 +52,7 @@ proc g++-dg-runtest { testcases flags default-extra-flags } {
 	    if { [llength $gpp_std_list] > 0 } {
 		set std_list $gpp_std_list
 	    } else {
-		set std_list { 98 14 17 2a }
+		set std_list { 98 14 17 20 }
 	    }
 	    set option_list { }
 	    foreach x $std_list {