consider making _mint private so that it is absolutely not used by implementers

[TimDaub]

• OZ's EIP-20 and EIP-721 encourage library users to override the internal mint functions and extend them for their purposes.
• But overriding mint in eip-4973 can break the entire contract's functionality and so we don't want nobody to override it
• here's an example of somone overriding mint: https://github.com/Shanmugadevan/POST/blob/0209610d89567fa058e96e3da9e66c90790d3b24/contracts/Soulbound.sol#L20