Bug 1923264 - Discard unsupported updates in selectUpdate. r=bytesized,nalexander

UpdateService.#selectUpdate currently chooses always the most recent
update even when unsupported and an older but supported one is
available.
This commit makes #selectUpdate discard the unsupported update if a
supported alternative is found.

Differential Revision: https://phabricator.services.mozilla.com/D224905