If you want to use Solidity 0.7, you may prefer to install @openzeppelin/contracts@solc-0.7
For the combination of upgradeable contracts and Solidity 0.7, use @openzeppelin/contracts-upgradeable@solc-0.7
From: OpenZeppelin Contracts 3.3
I would go through all of the reported issues/warnings and either resolve or decide why it doesn't need to be resolved. I would suggest documenting this for your community.
I believe the warning about the gap variables is a bug in Slither. There is no shadowing because the variables are private. Unless they consider that shadowing as well? @dmdv Do you want to report this to Slither or should we do it?