Use correct Nu6 constants

[alexeykoren]

• Use correct Nu6 constants
• Disable Nu7 activation for the time being

[what-the-diff]

PR Summary

• Update of the block height for network upgrade
  The block height, which is the position of a particular transaction in the blockchain, for NetworkUpgrade::Nu6 has been increased from 1_687_106 to 2_726_400. This adjustment will affect the order and positioning of transactions in the network.

• Setting network upgrade to None
  The NetworkUpgrade::Nu7 was set to None for all the Main, Test, and Regtest networks. This means this particular network upgrade won't have any impact on these specified networks.

• Modification in tests for new block height
  The integrated test suite has been updated. Now, it checks whether the new block height for NetworkUpgrade::Nu6 is functioning as expected.

• Change in the local consensus tests
  The local consensus tests now have a new condition in place. They ensure that the NetworkUpgrade::Nu7 is not currently active, thereby helping to maintain stability in the system.

• Transaction builder update
  The transaction builder was changed to retrieve the activation height (the point at which a particular change in the system becomes active) for NetworkUpgrade::Nu6 instead of Nu7. This implies that the 'Nu6' upgrade will be considered over the 'Nu7' when building transactions.

These changes help in improving transaction sequencing and making sure the related tests are updated to reflect these changes. It further ensures that certain network updates don't interfere with our current system. This contributes to improved system stability and efficiency.