new-committee: use committee cold key

[smelc]

Changelog

- description: |
    Make the `new-committee` command reach its final API (see [here](https://github.com/input-output-hk/cardano-cli/issues/300#issuecomment-1731004496))
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - improvement    # QoL changes e.g. refactoring
  - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

⚠️ Depends on these PRs to be merged first 🚧

• cardano-api-internal: ProposeNewCommittee: StakeKey -> CommitteeColdKey cardano-api#264
• https://github.com/input-output-hk/cardano-haskell-packages/pull/510
• Other PR to update to cardano-api 8.21.0.0 (or possibly 307 will replace this one altogether)

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• The change log section in the PR description has been filled in
• NA New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • round trip tests
  • integration tests
    See Running tests for more details
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-8.10.7 and ghc-9.2.7
• The changelog section in the PR is updated to describe the change
• Self-reviewed the diff

[smelc]

Closing in favor of #304