Make CPAN upstream for File::Temp

It was set to undef, which meant it hadn’t been discussed.  In actuality,
it is actively maintained on CPAN.  See, for instance:

    https://rt.cpan.org/Ticket/Display.html?id=75077#txn-1038945

So this brings Maintainers.pl closer in line with reality.

diff --git a/Porting/Maintainers.pl b/Porting/Maintainers.pl
index 928dd63..ee8da8a 100755 (executable)
--- a/Porting/Maintainers.pl
+++ b/Porting/Maintainers.pl
@@ -841,7 +841,7 @@ use File::Glob qw(:case);
                 misc/results.txt
                 ),
         ],
-        'UPSTREAM' => undef,
+        'UPSTREAM' => 'cpan',
     },
 
     'FileCache' => {