Update dependency rexml to v3.3.9 (#510)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

v2.4-non-rails/Gemfile.lock

@@ -42,7 +42,7 @@ GEM
     mini_portile2 (2.8.7)
     minitest (5.25.1)
     rake (13.2.1)
-    rexml (3.3.8)
+    rexml (3.3.9)
     rspec (3.13.0)
       rspec-core (~> 3.13.0)
       rspec-expectations (~> 3.13.0)