Downgrade libsodium to 1.0.20 to fix arm64 compile issue

Version 1.0.21 which we shipped with 6.2.3 has this bug, reported
by PhotoJim at https://bugs.unrealircd.org/view.php?id=6615.

And yes, libsodium also has this weird -stable thing, which does
have the fix, but that's basically just a snapshot of their git
version, it's a .tar.gz that gets updated every X time and it does
not have a GPG signature, while I have the policy nowadays to
verify GPG signatures for libraries we ship. So I am option to just
downgrade a version, for now, which is fine since we shipped with
1.0.20 for quite some time until recently.

configure

@@ -8984,7 +8984,7 @@ fi
 if test "$has_system_sodium" = "no"
 then :
 
-sodium_version="1.0.21"
+sodium_version="1.0.20"
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: extracting sodium library" >&5
 printf "%s\n" "extracting sodium library" >&6; }
 cur_dir=`pwd`