Add warning when a cloud block is present in a child module

[SarahFrench]

Currently users only see warnings about ignored backend blocks in child modules, and there isn't an equivalent warning about cloud blocks. This PR (#29826) introduced the testdata/nested-cloud-warning testing fixtures for this, but that PR doesn't appear to include a test that uses them.

This PR adds the missing warning, and adds a new test asserting the warning is present.

Target Release

N/A

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[github-actions]

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