Update dependency react-i18next to v15

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
react-i18next	^14.0.5 -> ^15.0.0

---

Release Notes

i18next/react-i18next (react-i18next)

### [`v15.0.0`](https://togithub.com/i18next/react-i18next/blob/HEAD/CHANGELOG.md#1500) [Compare Source](https://togithub.com/i18next/react-i18next/compare/v14.1.3...v15.0.0) - use optional chaining, nullish coalescing and nullish coalescing assignment [1774](https://togithub.com/i18next/react-i18next/pull/1774) - Build config and optimizations [1769](https://togithub.com/i18next/react-i18next/pull/1769) - some dependency updates [1768](https://togithub.com/i18next/react-i18next/pull/1768) - use modern hasLoadedNamespace code (now requires at least i18next > v19.4.5 (introduced in june 2020))

---

Configuration

📅 Schedule: Branch creation - "before 07:00 on Thursday" in timezone Europe/Oslo, Automerge - At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

---

• [ ] If you want to rebase/retry this PR, check this box

---

This PR was generated by Mend Renovate. View the repository job log.