Update ProposalProcedure return address to be a RewardAcnt

[teodanciu]

Description

Closes #3594

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[teodanciu]

I assume scriptsNeeded doesn't have to be updated for this, please correct me if I'm wrong

[lehins]

During a meeting yesterday we figured out a discrepancy. This has to be a RewardAcnt, not just a Credential

[teodanciu]

I updated it to be a RewardAcnt, however, I haven't used the Network anywhere.
I assume it will be needed (otherwise why have it), but not sure if at this point. Maybe in the way we insert in UMap in Epoch rule?

[lehins]

We'll only need to validate that the NetworkId matches the network we are running. That's all.

See this ticket I created: #3603

[lehins]

Great. Thank you!