[ACP-77] Adjust nomenclature due to renaming of Avalanche Subnets -> Avalanche L1s

[meaghanfitzgerald]

As stated in this Avalanche Academy Guide: Avalanche Subnets have been rebranded as Avalanche L1s.

To avoid confusion, the spec has been revised to account for this, including the following changes:

ConvertSubnetTx -> ConvertL1Tx
RegisterSubnetValidatorTx -> RegisterL1ValidatorTx
RegisterSubnetValidatorMessage -> RegisterL1ValidatorMessage
SetSubnetValidatorWeightTx -> SetL1ValidatorWeightTx
DisableValidatorTx -> DisableL1ValidatorTx
SubnetValidatorRegistrationMessage -> L1ValidatorRegistrationMessage
IncreaseBalanceTx -> IncreaseL1ValidatorBalanceTx 

To Review:

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Nuance:

• Subnet validator continues to refer to all validators created and controlled by Subnet Owners pre-Etna Upgrade, controllable by the AddSubnetValidatorTx and RemoveSubnetValidatorTx.
• CreateSubnetTx is a pre-Etna transaction whose functionality and nomenclature will not change within this spec.
• SubnetID is the transaction hash produced by a successful CreateSubnetTx and a parameter required by some of the new transaction types. This will not be renamed.