Make improvements based on OpenZeppelin's updates to their MetaTransactions implementation

[SKYBITDev3]

OpenZeppelin have released v5 of their contracts: https://github.com/OpenZeppelin/openzeppelin-contracts/releases/tag/v5.0.0

It includes a number of updates to their MetaTransactions implementation, which may be good to use to improve OpenGSN's implementation.

OpenGSN's implementation is still preferred because OpenZeppelin makes forwarder immutable.