Make epoch 3.3 height required in stacks-node config

[hugoclrd]

In #6584, the epoch 3.3 height was added to the stacks-node config. But not required.

I should be required in the 3.3 release

https://github.com/stacks-network/stacks-core/pull/6584/files/3dd4ac39dae880a3d7c24876813d0aec172454b6#diff-b962ae0660fbed5e014fb65d01337236d279da63daf88b7126d3921f6e3bb113R754

[brice-stacks]

@hugoclrd, please confirm that #6633 closes this issue. Thanks!