Bump @mui/material from 5.15.19 to 5.15.20

Bumps [@mui/material](https://github.com/mui/material-ui/tree/HEAD/packages/mui-material) from 5.15.19 to 5.15.20.
- [Release notes](https://github.com/mui/material-ui/releases)
- [Changelog](https://github.com/mui/material-ui/blob/v5.15.20/CHANGELOG.md)
- [Commits](https://github.com/mui/material-ui/commits/v5.15.20/packages/mui-material)

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

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

package.json

@@ -12,7 +12,7 @@
     "@gatsbyjs/reach-router": "^2.0.1",
     "@mdx-js/react": "^2.3.0",
     "@mui/lab": "^5.0.0-alpha.97",
-    "@mui/material": "^5.15.19",
+    "@mui/material": "^5.15.20",
     "@mui/x-data-grid": "^7.6.2",
     "@mui/x-date-pickers": "^7.6.2",
     "@popperjs/core": "^2.11.8",