migrations: fixup migration error on Postgres 10.x.

`CREATE OPERATOR` only accepts the `FUNCTION` argument since Postgres 11
or higher.

db/migrate/20210110015410_add_reverse_regex_operator.rb

@@ -1,7 +1,7 @@
 class AddReverseRegexOperator < ActiveRecord::Migration[6.1]
   def up
     execute "CREATE FUNCTION reverse_textregexeq (text, text) RETURNS boolean LANGUAGE sql IMMUTABLE PARALLEL SAFE AS $$ SELECT textregexeq($2, $1); $$"
-    execute "CREATE OPERATOR ~<< (FUNCTION = reverse_textregexeq, leftarg = text, rightarg = text)"
+    execute "CREATE OPERATOR ~<< (PROCEDURE = reverse_textregexeq, leftarg = text, rightarg = text)"
   end
 
   def down