Update Spell Mainnet Repo checklist to consider the new voting portal and poll repos

[0x3phemeralsoul]

• As of next week polls will be posted to vote.sky.money, not vote.makerdao.com
• poll files will be hosted on the makerdao/polls repo
• executive vote files will be hosted on the makerdao/executive-votes repo.

The spell checklists specify the makerdao/community repo and vote.makerdao.com as sources of truth

Acceptance

• Checklists are updated to reflect the new repos and website URL