Issue #3590655: Fix Canvas 1.4.x compatibility — remove $ref from test component

The justinrainbow/json-schema library sets id: on any schema resolved from a $ref URI. Canvas 1.0.x accepted this silently; Canvas 1.4.x upgraded AJV to v8, which throws "NOT SUPPORTED: keyword id" when it encounters the draft-04 id keyword.

The uncaught AJV exception also broke multi-value field rendering, causing array props to only allow one selection.

Fix: replace the $ref approach in the test-multi-nodes component with a direct x-entity-type: taxonomy_term annotation, which is not affected. Add a compatibility note to the README explaining the root cause and recommending x-entity-type for Canvas 1.4.x+.

Note: this is a Canvas core bug — PropShape::normalizePropSchema() already strips id (see its inline comment), but stripNonStandardJsonSchemaKeys() does not. A separate Canvas issue should be filed to fix it there as well.