Add HARD_FORK_HEIGHT to initial-config.yaml for testneta

[TheLastCicada]

Purpose:

TestnetA does not have a hard-fork height specified and can therefore not successfully farm plots with the latest version of Chia. This adds a hard fork height to the default config, which is also being applied on any existing testneta infrastructure.

[arvidn]

in order to make this "just work" for existing farmers and nodes that already have a config.yaml, you also need to introduce this override if it doesn't exist in the config file. here: https://github.com/Chia-Network/chia-blockchain/blob/main/chia/consensus/default_constants.py#L112

[cmmarslender]

in order to make this "just work" for existing farmers and nodes that already have a config.yaml, you also need to introduce this override if it doesn't exist in the config file. here: main/chia/consensus/default_constants.py#L112

I think we control the majority of the nodes on the network, so we had already added it to those node's configs, but if you think its ok to have something in default constants as well to make this work a bit better, we could also add something there.