Change Spectral lint to fail on errors only, not warnings

All components are flagged as "unused" because paths reference them
via $ref files rather than inline. These are false positives that
will resolve as the spec matures. Warnings are still reported.

Author: bfowle

.github/workflows/validate-openapi.yml

@@ -27,4 +27,4 @@ jobs:
           node-version: "22"
 
       - name: Run Spectral lint
-        run: npx --yes @stoplight/spectral-cli lint spec/openapi.yaml --fail-severity warn
+        run: npx --yes @stoplight/spectral-cli lint spec/openapi.yaml --fail-severity error