governance: unify final proposal state as Completed for proposal with no transaction inserted

[ochaloup]

The governance proposal may be "locked" in state Succeeded and won't be moved to Completed if one does not insert any transaction to be executed while want to use deny options.

When having a proposal withoug the transactions to be executed while creating the proposal with use_deny_option = false then the proposal finishes in Completed state after FinalizeVote ix.
When there is the use_deny_option = true then the proposal ends in Succeeded state. As there is no transaction to execute it's unclear to call ExecuteTransaction wish some null data (if it's even possible).
It should be clear and easy to move such a proposal to final state Completed.

The proposal finalize vote use deny vs. not use deny:

solana-program-library/governance/program/src/state/proposal.rs

Line 415 in aaaf537

if self.deny_vote_weight.is_none() {

solana-program-library/governance/program/src/state/proposal.rs

Line 395 in aaaf537

OptionVoteResult::Succeeded

Current behaviour of Succeeded to Completed state change:

• useDenyOption == false; no transaction can be executed and the FinalizeVote switches to end state ProposalState::Completed
• useDenyOption == true and no transactions; the FinalizeVote switches to ProposalState::Succeeded and it's not possible (or unclear how) to move to Completed
• useDenyOption == true + transactions; the FinalizeVote switches to ProposalState::Succeeded and then ExecuteTransaction will run stored transactions and at the end moved to ProposalState::Completed

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As per discussion with @SebastianBor, see at
https://discord.com/channels/910194960941338677/910566058740568094/1035192118349668392
It could be a solution to add a new instruction CompleteProposal. If conditions are met then it would transition a proposal to a Completed state. It would work in the following use cases

1. It would allow to transition past proposal which are stuck in Succeeded state
   2) It would allow to transition non executable proposals (ExecutingWithErrors) into Completed state after let's say 1 year of being in that state to make sure they are non actionable any longer