Switch to using TxCert type family

[lehins]

Description

Instead of using the ConwayTxCert concrete type. This will be helpful when we start defining a new era.

Fixes #4899

Checklist

• Commits in meaningful sequence and with useful messages
• Tests added or updated when needed
• CHANGELOG.md files updated for packages with externally visible changes
  
  New section is never added with the code changes. (See RELEASING.md)
• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the
  versioning process.
• Version bounds in .cabal files updated when necessary
  
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Code formatted (use scripts/fourmolize.sh)
• Cabal files formatted (use scripts/cabal-format.sh)
• hie.yaml updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[teodanciu]

Looks good to me