refactor: Bump rexml from 3.3.6 to 3.3.9 (#1817)

Author: dependabot[bot]

Gemfile.lock

@@ -92,15 +92,13 @@ GEM
     public_suffix (4.0.7)
     rake (13.0.6)
     redcarpet (3.6.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rouge (2.0.7)
     ruby-macho (1.4.0)
     sassc (2.4.0)
       ffi (~> 1.9)
     sqlite3 (1.7.3)
       mini_portile2 (~> 2.8.0)
-    strscan (3.1.0)
     thread_safe (0.3.6)
     typhoeus (1.4.1)
       ethon (>= 0.9.0)