build: bump @stoplight/spectral-core from 1.20.0 to 1.21.0

Bumps [@stoplight/spectral-core](https://github.com/stoplightio/spectral) from 1.20.0 to 1.21.0.
- [Release notes](https://github.com/stoplightio/spectral/releases)
- [Changelog](https://github.com/stoplightio/spectral/blob/develop/CHANGELOG.md)
- [Commits](https://github.com/stoplightio/spectral/compare/@stoplight/spectral-core-1.20.0...@stoplight/spectral-core-1.21.0)

---
updated-dependencies:
- dependency-name: "@stoplight/spectral-core"
  dependency-version: 1.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

package-lock.json

@@ -31,7 +31,7 @@
     "ruleset"
   ],
   "dependencies": {
-    "@stoplight/spectral-core": "^1.20.0",
+    "@stoplight/spectral-core": "^1.21.0",
     "@stoplight/spectral-documentation": "^1.3.1",
     "lodash": "^4.17.23"
   },