XML response body formatting performance

[shockey]

Description

This PR removes our internal formatXml utility function in favor of xml-but-prettier.

This change cuts our time spent rendering a 2MB XML payload from 78s to 1.3s.

Motivation and Context

Fixes #3832!

How Has This Been Tested?

• I tested an XML Try-It-Out endpoint before and after the change.
• xml-but-prettier has its own tests.

Types of changes

• No code changes (changes to documentation, CI, metadata, etc)
• Dependency changes (any modification to dependencies in package.json)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds 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 added tests to cover my changes.
• All new and existing tests passed.