BIP-2: Reduce amount of BITs required to submit governance proposal

Background: The current proposal creation requires 10M BITs too high, and no new proposal has been created so far.

Lowering the threshold is conducive to active communities, making it easier for people with ideas to create proposals, and allowing more people to participate in building a BitDao.

Proposal: According to BitDAO Token Holders, there are very few addresses with a balance higher than 10M, and most of the BIT is in the contract address. So I think the threshold should be 5M or lower.