Slightly broken Markdown formatting when used with --desc

[woodruffw]

This is a relatively minor bug: the combination of --desc and --format=markdown might result in a broken Markdown table, since --desc can include multiple lines of embedded Markdown from the vulnerability service.

For example, from the self-test on our GitHub Action: https://github.com/trailofbits/gh-action-pip-audit/actions/runs/2592038764

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I see two possible solutions (but there are probably others!):

• Forbid the combination of --desc and --format=markdown
  • Pros: Arguably the most correct, since --desc is explicitly allowed to contain Markdown per the OSV spec on vulnerability descriptions
  • Cons: Makes the Markdown format a little less useful.
• "Collapse" the Markdown into a single line
  • Pros: Preserves descriptions in the Markdown table, which aren't multiple lines 99% of the time.
  • Cons: Will slightly mis-render any Markdown descriptions that contain multiple lines, if those multiple lines are semantically important (e.g. headings)

I'm slightly leaning towards the second.

[woodruffw]

This will be partially alleviated (but not fixed) by pypi/warehouse#11734, since we'll be able to use the summary instead of the full description, when present.

[woodruffw]

No longer blocked upstream; I can make the needed changes here today.