fix(RSS Feed Trigger Node): Use newest date instead of first item for new items

[meownoid]

Summary

Describe what the PR does and how to test. Photos and videos are recommended.

This PR fixes issue described in the #8856 when RSS Feeds where latest element is not last in the feed leads to infinitely repeating triggers. Original trigger node does not have tests and I'm not sure how to create one from scratch.

Related tickets and issues

Include links to Linear ticket or Github issue or Community forum post. Important in order to close automatically and provide context to reviewers.

#8856

Review / Merge checklist

• [x] PR title and summary are descriptive. Remember, the title automatically goes into the changelog. Use (no-changelog) otherwise. (conventions)
• [ ] Docs updated or follow-up ticket created.
• [ ] Tests included.

  A bug is not considered fixed, unless a test is added to prevent it from happening again. A feature is not complete without tests.

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