chore: bump up react-i18next version to v15 (#7529)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [react-i18next](https://togithub.com/i18next/react-i18next) | [`^14.1.0` -> `^15.0.0`](https://renovatebot.com/diffs/npm/react-i18next/14.1.3/15.0.0) | [![age](https://developer.mend.io/api/mc/badges/age/npm/react-i18next/15.0.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://developer.mend.io/api/mc/badges/adoption/npm/react-i18next/15.0.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://developer.mend.io/api/mc/badges/compatibility/npm/react-i18next/14.1.3/15.0.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/react-i18next/14.1.3/15.0.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>i18next/react-i18next (react-i18next)</summary>

### [`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))

</details>

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packages/frontend/i18n/package.json

@@ -23,7 +23,7 @@
     "dayjs": "^1.11.11",
     "i18next": "^23.11.1",
     "react": "^18.2.0",
-    "react-i18next": "^14.1.0",
+    "react-i18next": "^15.0.0",
     "undici": "^6.12.0"
   },
   "devDependencies": {