Bump rexml from 3.3.6 to 3.3.9

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.6 to 3.3.9.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.3.6...v3.3.9)

---
updated-dependencies:
- dependency-name: rexml
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -426,8 +426,7 @@ GEM
     regexp_parser (2.8.2)
     reline (0.4.2)
       io-console (~> 0.5)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rspec-core (3.10.2)
       rspec-support (~> 3.10.0)
     rspec-expectations (3.10.2)
@@ -523,7 +522,6 @@ GEM
       activesupport (>= 5.2)
       sprockets (>= 3.0.0)
     stringio (3.1.0)
-    strscan (3.1.0)
     sys-uname (1.2.2)
       ffi (~> 1.1)
     sysexits (1.2.0)