Bug 2019091 - Reset Nimbus databases that are on v1

[freshstrangemusic]

There was previously a bug where Nimbus databases that were stuck at v1 would mistakenly only ever update to v2 but then write that they were v3 to their database, preventing them from ever migrating correctly to v3. This is unlikely to happen because the migration from v1 to v2 was 5 years ago, so a client would have to have been installed but unused for that entire period.

In light of this, we are removing the v1 to v2 migration and resetting all databases that are < v2. This is unlikely to affect any real users.

This does not fix the general case issue if we added an additional migration (i.e., v3 to v4), but that will be resolved in bug 1997631, which is refactoring the migration system and adding telemetry.

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