build(deps): bump rexml from 3.2.5 to 3.2.8 in /charts

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](ruby/rexml@v3.2.5...v3.2.8)

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

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

charts/Gemfile.lock

@@ -13,7 +13,9 @@ GEM
       matrix (~> 0.4)
       pdf-core (~> 0.10.0)
       ttfunk (~> 1.8)
-    rexml (3.2.5)
+    rexml (3.2.8)
+      strscan (>= 3.0.9)
+    strscan (3.1.0)
     ttfunk (1.8.0)
       bigdecimal (~> 3.1)