Add genesis information to freezer metadata file and update verification to use it

[fjl]

When running Geth with pruned history, the freezer validation will fail at startup when it attempts to compare the genesis block hash to the block stored in freezer. We have this check to ensure the user doesn't configure a freezer directory for the wrong network.

The metadata file (last updated in #30392) should be changed to include the genesis hash. That way, we can perform the validation check without actually storing the genesis block.

[fjl]

Turns out we don't need to do this since we keep the headers even in pruned mode.