Ensure version property is required when present in legacy telemetry

[sean-rose]

Checklist for reviewer:

• Commits should reference a bug or github issue, if relevant (if a bug is referenced, the pull request should include the bug number in the title)
• If adding a new field, the field should have a description (see Bug 1652842 - Add a blocklist field so the environment schema works with pre- and post-bug-1647225 data #576 for an example)
• If coming from a fork, run integration tests: ./.github/push-to-trigger-integration <username>:<branchname>

For glean changes:

• Update templates/include/glean/CHANGELOG.md

For modifications to schemas in restricted namespaces (see CODEOWNERS):

• Follow the change control procedure

[BenWu]

I don't think this is necessary because the decoder drops these pings before they get to schema validation but it also doesn't hurt to be more explicit in the schema. I don't have a problem with this especially if it's needed for the other PR but it'd be good to have @akkomar to review

[sean-rose]

especially if it's needed for the other PR

It isn't needed for #811 as I've already made adjustments there to more gracefully handle version properties that might be configured to allow nulls (though that should never be the case in the actual data due to how ingestion works as you've pointed out).

So this is more about making the affected schemas more reflective of reality, consistent with the other legacy schemas, and enforcing that consistency for legacy schemas going forward.