fix: upgrade @mui/material from 5.15.20 to 5.15.21

Snyk has created this PR to upgrade @mui/material from 5.15.20 to 5.15.21.

See this package in npm:
@mui/material

See this project in Snyk:
https://app.snyk.io/org/sumn2u/project/0638e038-16c8-4368-8094-0428fd598831?utm_source=github&utm_medium=referral&page=upgrade-pr

Author: snyk-bot

client/package-lock.json

@@ -14,7 +14,7 @@
     "@fortawesome/free-solid-svg-icons": "^6.5.2",
     "@fortawesome/react-fontawesome": "^0.2.2",
     "@mui/icons-material": "^5.15.20",
-    "@mui/material": "^5.15.20",
+    "@mui/material": "^5.15.21",
     "@mui/x-charts": "^7.8.0",
     "axios": "^1.7.2",
     "cash-dom": "^8.1.5",