Update REXML to 3.4.2 or later to fix DOS vulnerability

The REXML gems from 3.3.3 to 3.4.1 have a DoS vulnerability when parsing XML containing multiple XML declarations: https://www.ruby-lang.org/en/news/2025/09/18/dos-rexml-cve-2025-58767/

We are currently using Fastlane that depends on this gem.

Start by checking how Fastlane brings in the REXML gem and where this project's dependency requirements are defined. Read the linked Ruby security advisory, then verify that the resolved REXML version is 3.4.2 or later and that the dependency checks pass.

Written by the indexing model from the issue text.