build(deps): bump rexml from 3.3.8 to 3.3.9 in /uaa/slate (#3100)

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.8 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@v3.3.8...v3.3.9)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

uaa/slate/Gemfile.lock

@@ -110,7 +110,7 @@ GEM
     rb-inotify (0.11.1)
       ffi (~> 1.0)
     redcarpet (3.6.0)
-    rexml (3.3.8)
+    rexml (3.3.9)
     rouge (3.30.0)
     sass (3.7.4)
       sass-listen (~> 4.0.0)