Use new `markdownlint` rules for emphasis- and strong-styles

[mcdonnnj]

🗣 Description

This pull request adds rules for MD049 and MD050, which were made available in 0.31.0 of our markdownlint hook. It also fixes the comment header for another rule in the same configuration.

💭 Motivation and context

Now that these rules are available we should enforce a specific style. I have chosen to use asterisks (*) due to general consensus about support. Per the CommonMark specification:

Many implementations have also restricted intraword emphasis to the * forms, to avoid unwanted emphasis in words containing internal underscores. (It is best practice to put these in code spans, but users often do not.)

🧪 Testing

Automated tests pass.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All new and existing tests pass.