These changes make the `OptimismMintableERC721Factory` in line
with the bedrock version of the `OptimismMintableERC20Factory`.
We might as well follow what we are doing with the new erc20
factory to keep things in line with each other.

These changes are very small, they just rename a function to
not follow the deprecated naming scheme, the function now returns
an address which makes testing/integration a bit easier and also
does a slight refactor to prevent the need to cast a type 3 times.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>