29717 wp check invalid utf8

[pbearne]

Trac ticket: 29717

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props pbearne.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

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The changes in this pull request can previewed and tested using a WordPress Playground instance.

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For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

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