cardano-testnet: declare supported cli/node versions

[palas]

Description

Declares which cardano-cli\ and cardano-node versions cardano-testnet supports,
via two zero-maintenance mechanisms:

1. README "Supported versions" section — states the single-release policy:
   use cli and node from the same release tarball as cardano-testnet.

2. Extended cardano-testnet version output — reports the cardano-api and
   cardano-cli versions the build was compiled against, using Cabal's
   auto-generated VERSION_cardano_api and VERSION_cardano_cli CPP macros.
   The cabal file remains the sole source of truth.

Also fixes the version banner which incorrectly said "cardano-node" instead of
"cardano-testnet".

Closes #6548

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Running tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
• 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-9.6 and ghc-9.12
• Self-reviewed the diff

[Jimbo4350]

We should implement a golden test for this.

[palas]

We should implement a golden test for this.

@Jimbo4350, I've added a golden test in the latest commit. But, in retrospective, I am not sure it was a good idea. Because it needs to be changed in each bump. And I already deleted two lines to avoid the git rev (which would need to be changed in every commit), and the architecture (which would just never match), as well as the cardano-node version