Migrate the MCP server to the MCP TypeScript SDK v2 (#56280)

This PR migrates the implementation of `npx convex mcp` from `@modelcontextprotocol/sdk` (v1) to `@modelcontextprotocol/server` (v2).

This follows the migration guide at https://ts.sdk.modelcontextprotocol.io/v2/migration/upgrade-to-v2.html

The v2 package requires Node 20, so Node 20 becomes the actual CLI minimum. This is fine, as Node.js 20 itself is EOL.

GitOrigin-RevId: 378b22e3d7c65d4fba0ed98fa3beae774817ee54