npm: bump @mui/system from 5.15.15 to 5.15.20 in /src/main/web

Bumps [@mui/system](https://github.com/mui/material-ui/tree/HEAD/packages/mui-system) from 5.15.15 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-system)

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

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

src/main/web/package.json

@@ -16,7 +16,7 @@
     "@lexical/table": "^0.15.0",
     "@lexical/utils": "^0.15.0",
     "@mui/material": "^5.15.19",
-    "@mui/system": "^5.15.15",
+    "@mui/system": "^5.15.20",
     "@react-buddy/ide-toolbox": "^2.4.0",
     "@react-buddy/palette-mui": "^5.0.1",
     "@react-oauth/google": "^0.12.1",