ci/request-reviews: lowercase handles consistently

This script assumed to get lowercased input before, but with the
addition of pinging maintainers that's not necessarily true anymore.

Since the checks for prAuthor and already-reviewed-by already lowercase,
make sure to lowercase the handles in the users array, too.

ci/request-reviews/process-reviewers.sh

@@ -24,7 +24,7 @@ trap 'rm -rf "$tmp"' exit
 
 declare -A users=()
 while read -r handle && [[ -n "$handle" ]]; do
-    users[$handle]=
+    users[${handle,,}]=
 done
 
 # Cannot request a review from the author