Assume a baseline of 3.3 for server protocol versions

3.3.0 was the first KRaft release that was deemed production-ready and also when KIP-778 (KRaft to KRaft upgrades) landed. Given that, it's reasonable for 4.x to only support upgrades from 3.3.0 or newer (the metadata version also needs to be set to "3.3" or newer before upgrading).