Bump rexml from 3.3.6 to 3.3.9 in /ios

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>

ios/Gemfile.lock

@@ -156,8 +156,7 @@ GEM
       trailblazer-option (>= 0.1.1, < 0.2.0)
       uber (< 0.2.0)
     retriable (3.1.2)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rouge (2.0.7)
     ruby2_keywords (0.0.4)
     rubyzip (2.3.0)
@@ -170,7 +169,6 @@ GEM
     simctl (1.6.8)
       CFPropertyList
       naturally
-    strscan (3.1.0)
     terminal-notifier (2.0.0)
     terminal-table (1.8.0)
       unicode-display_width (~> 1.1, >= 1.1.1)