Upgrade mysql2

[CarolineDenis]

Upgrade mysql2 to fix 3 Dependabot alerts in app/package-lock.json

Upgrade mysql2 to version 3.9.4 or later. For example:

"dependencies": {
"mysql2": ">=3.9.4"
}
"devDependencies": {
"mysql2": ">=3.9.4"
}