Fix HTML being rendered in mirror error message

What does this MR do?

In Rails 6, more String manipulation methods are now HTML-safe aware. https://github.com/rails/rails/pull/33990

This means, when doing the #insert here it first escapes the HTML before inserting into the HTML-safe string

This is similar to !20426 (merged)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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