Test backwards compatibility in golden tests

Our golden tests check whether encoding the example value still gives the same expected bytes.

Sometimes we change the encoding format of, e.g., the ledger state, but make the decoder backwards compatible with the old format (we use [`Ouroboros.Consensus.Util.Versioned`](https://github.com/input-output-hk/ouroboros-network/blob/3d990f8358dd500f78e2932e1a6ace9d5b982650/ouroboros-consensus/src/Ouroboros/Consensus/Util/Versioned.hs) for that when possible). See [this example](https://github.com/input-output-hk/ouroboros-network/blob/3d990f8358dd500f78e2932e1a6ace9d5b982650/ouroboros-consensus-shelley/src/Ouroboros/Consensus/Shelley/Ledger/Ledger.hs#L527-L533). It would be good to test this using the existing golden test infrastructure we have. In the test, we would check whether decoding the old bytestring still results in the same example value.

Start by examining the existing golden test infrastructure and the Versioned.hs utility linked in the issue, then compare the compatibility example in Shelley/Ledger/Ledger.hs. Add coverage that decodes the old bytestring and verifies it produces the same example value.

Written by the indexing model from the issue text.