chore(deps): bump rexml from 3.2.5 to 3.2.8 in /tests/ruby (#743)

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.5 to 3.2.8.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](https://github.com/ruby/rexml/compare/v3.2.5...v3.2.8)

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

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

tests/ruby/Gemfile.lock

@@ -24,7 +24,8 @@ GEM
     pg (1.3.2)
     rainbow (3.1.1)
     regexp_parser (2.3.1)
-    rexml (3.2.5)
+    rexml (3.2.8)
+      strscan (>= 3.0.9)
     rspec (3.11.0)
       rspec-core (~> 3.11.0)
       rspec-expectations (~> 3.11.0)
@@ -50,6 +51,7 @@ GEM
     rubocop-ast (1.17.0)
       parser (>= 3.1.1.0)
     ruby-progressbar (1.11.0)
+    strscan (3.1.0)
     toml (0.3.0)
       parslet (>= 1.8.0, < 3.0.0)
     toxiproxy (2.0.1)