[Docs][8.17.0] Reverting back to ILM as default

[lahsivjar]

In 8.17.0, we are reverting to ILM as default for managing lifecycle (ref). We should change the wordings in this doc to reflect the updates. Note that, we don't need to change the prefer_ilm flag since true is what we want but that flag won't be necessary now since prefer_ilm will be set to true by default.