Set default network hardfork for each network instead of for the entire application

[tenthirtyone]

The latest hardfork to land on mainnet, gray glacier, does not exist on any testnet. Setting the default hardfork in ganache to gray glacier breaks testnet forking unless a valid hardfork, e.g. london is supplied at startup.

[MicaiahReid]

To expand on this, if a user starts ganache with ganache --fork mainnet, the default hardfork should be grayGlacier. However, if ganache --fork sepolia is used, the default hardfork should be london (because grayGlacier has not been implemented on any of the testnets).