changing windows_ubr to be type string

[erchen03]

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]

We can't trivially change the data type of a field after it's added to the bigquery schema. We would need to do something like windows_ubr2 which would require a client-side release or some special case in ingestion. I'll see if we have some precedent for doing this