db.voters could be more clearly named

[dsernst]

This collection name was originally picked early on.

But now that we also have votes-pending, and votes-invalidated, it would be more clear if this collection was called voters-approved or similar, instead of just voters.

Also related to https://github.com/siv-org/siv/issues/172.

[dsernst]

Three step process to migrate:

• [ ] 1. Update logic for all new ones moving forward
• [ ] 2. Update endpoints to expect it
• [ ] 3. Script to move old ones over