chore(deps-dev): bump @storybook/addon-a11y from 5.3.21 to 6.5.15

Bumps [@storybook/addon-a11y](https://github.com/storybookjs/storybook/tree/HEAD/addons/a11y) from 5.3.21 to 6.5.15.
- [Release notes](https://github.com/storybookjs/storybook/releases)
- [Changelog](https://github.com/storybookjs/storybook/blob/v6.5.15/CHANGELOG.md)
- [Commits](https://github.com/storybookjs/storybook/commits/v6.5.15/addons/a11y)

---
updated-dependencies:
- dependency-name: "@storybook/addon-a11y"
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

packages/core/package.json

@@ -3,7 +3,7 @@
   "version": "0.29.1",
   "private": true,
   "devDependencies": {
-    "@storybook/addon-a11y": "^5.3.18",
+    "@storybook/addon-a11y": "^6.5.15",
     "@storybook/addon-docs": "^5.3.18",
     "@storybook/addon-knobs": "^5.3.18",
     "@storybook/addon-notes": "^5.3.18",