Update: react-i18next to 12.1.5

[mandotpro]

Hey just upgrade to 12.1.5 to resolve i18next/react-i18next#1543
Included in i18next/react-i18next#1606

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[adrai]

Since it's just a peer dep, I think there's no need to publish a new version.