Systematic changes to generated OSVs should automatically bump the modified time

[mbauman]

This is a bit of an oblique case, but changes like #102 will meaningfully change the exported advisories without automatically bumping the modification dates.

Not sure what to do here; perhaps we just need an action that manually bumps all timestamps that @jlsec-bot could run on PRs at request or something. We could just update the modified timestamps in the Markdown/TOML, and that will be enough to get the auto-timestamping to do its thing properly once again.

[mbauman]

Or, wait, I'm confused now because after merging #105, the action bumped all the timestamps. That shouldn't have happened... well maybe it should, but it's not doing what I expected.

[mbauman]

OK, the overzealous timestamp updates are addressed in #107. Now we should see the behaviors that I expect; namely that systemic changes like #102 won't update timestamps without manual intervention.

[mbauman]

The update-and-export action itself should be able to detect this situation.

We could start by erroring if the OSV files change beyond what is expected.

[mbauman]

Great; with #108, we at least have CI detecting and erroring if this happens before the OSVs get pushed. Adding purls to the output in #102 was a good testbed.