chore(deps): bump @mui/material from 5.14.20 to 5.15.12 in /website

Bumps [@mui/material](https://github.com/mui/material-ui/tree/HEAD/packages/mui-material) from 5.14.20 to 5.15.12.
- [Release notes](https://github.com/mui/material-ui/releases)
- [Changelog](https://github.com/mui/material-ui/blob/master/CHANGELOG.md)
- [Commits](https://github.com/mui/material-ui/commits/v5.15.12/packages/mui-material)

---
updated-dependencies:
- dependency-name: "@mui/material"
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

website/package.json

@@ -20,7 +20,7 @@
     "@astrojs/node": "^6.1.0",
     "@emotion/react": "^11.11.4",
     "@headlessui/react": "^1.7.18",
-    "@mui/material": "~5.14.20",
+    "@mui/material": "~5.15.12",
     "@mui/x-date-pickers": "^6.19.6",
     "@svgr/core": "^8.1.0",
     "@svgr/plugin-jsx": "^8.1.0",