Update Patch and Upgrade docs to include gitlab-elasticsearch-indexer

We were advised to use the experimental indexer, but currently which version to use / the upgrade path is unclear as currently the gitlab-elasticsearch-indexer isn't part of either the upgrade or patch docs.

Even though it's an optional feature, I think it should be added to those docs. In a similar fashion to Gitaly before it was mandatory.

This was previously discussed in support ticket: 82921