Bump rexml to 3.3.6 on example apps to avoid potential DoS vulnerability

[sbarrio]

What does this PR do?

Dependabot has notified us of a security issue that could cause Denial of Service attacks related to rexml on both of our example apps. Bumping it to 3.3.6 solves the problem.

From https://github.com/DataDog/dd-sdk-reactnative/security/dependabot/10

The REXML gem before 3.3.6 has a DoS vulnerability when it parses an XML that has many deep elements that have same local name attributes.

If you need to parse untrusted XMLs with tree parser API like REXML::Document.new, you may be impacted to this vulnerability. If you use other parser APIs such as stream parser API and SAX2 parser API, this vulnerability is not affected.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)
• If this PR is auto-generated, please make sure also to manually update the code related to the change

[datadog-datadog-prod-us1]

Datadog Report

Branch report: sbarrio/fix/security-dependency-rexml
Commit report: 53d219a
Test service: dd-sdk-reactnative

✅ 0 Failed, 663 Passed, 1 Skipped, 3.75s Total Time