Use `getZIndex` for all z-index values

We have a utility function [`getZIndex`](https://github.com/guardian/dotcom-rendering/blob/c272244af161813864a5b88e27086bcdec577001/dotcom-rendering/src/lib/getZIndex.ts#L120-L124) that allows us to deterministically sequence `z-index` values for all items in the declared list it consumes.

However there are still many [examples](https://github.com/search?q=repo%3Aguardian%2Fdotcom-rendering+%22z-index%3A+%22&type=code&p=2) of raw z-index values being set in the DCAR codebase which should be moved to use the `getZIndex` function.

This will allow us to confidently create z-index values and avoid mistakes where a change to underlying z-index values cause items to not stack correctly.

For example: #16140

Start with dotcom-rendering/src/lib/getZIndex.ts and the repository code search for raw `z-index` declarations. Review the example in issue #16140, then inspect each relevant occurrence; done means the DCAR examples use the utility rather than raw values.

Written by the indexing model from the issue text.