GEP: Rename Core and Extended support levels

**What would you like to be added**:

This GEP proposes that we rename Core and Extended support levels to Required and Optional, in the interest of increasing clarity and preventing confusion.

**Why this is needed**:

We already see confusion in talking about Core and Extended (particularly confusing Extended with Experimental), and we almost always use Required and Optional to explain the terms. So we should just rename.

The issue names no files, tests, or entry points. Start by locating all references to the Core and Extended support levels in the Gateway API repository and review the existing GEP terminology. Done means the support levels are consistently renamed to Required and Optional without confusing them with Experimental.

Written by the indexing model from the issue text.