Targeted utf8mb3 to utf8mb4 fix for query_doc_pairs.document_field

[epugh]

Description

This just makes the minimal change to support emoji.

Motivation and Context

This is a targted fix for #1013 issue, that doesn't try to clean up the overall schema....

It is an alternative to #1016 which I am more nervous about rolling out!

How Has This Been Tested?

Screenshots or GIFs (if appropriate):

Types of changes

• [] Bug fix (non-breaking change which fixes an issue)
• [] Improvement (non-breaking change which improves existing functionality)
• [] New feature (non-breaking change which adds new functionality)
• [] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [] My code follows the code style of this project.
• [] My change requires a change to the documentation.
• [] I have updated the documentation accordingly.
• [] I have read the CONTRIBUTING document.
• [] I have added tests to cover my changes.
• [] All new and existing tests passed.