Replace deprecated i18next-xhr-backend with i18next-http-backend

[ziegenberg]

The new i18next-http-backend is based on the deprecated i18next-xhr-backend and can mostly be used as a drop-in replacement.

[github-actions]

Use docker or podman to test this pull request locally.

Run test server using develop.opencast.org as backend:

podman run --rm -it -p 127.0.0.1:3000:3000 ghcr.io/opencast/opencast-admin-interface:pr-461

Specify a different backend like stable.opencast.org:

podman run --rm -it -p 127.0.0.1:3000:3000 -e PROXY_TARGET=https://stable.opencast.org ghcr.io/opencast/opencast-admin-interface:pr-461

It may take a few seconds for the interface to spin up.
It will then be available at http://127.0.0.1:3000.
For more options you can pass on to the proxy, take a look at the README.md.

[github-actions]

This pull request has conflicts ☹
Please resolve those so we can review the pull request.
Thanks.

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/461/2024-05-31_13-12-13/ .
It might take a few minutes for it to become available.

[ziegenberg]

Rebased onto current main

[github-actions]

This pull request is deployed at test.admin-interface.opencast.org/461/2024-05-31_13-34-41/ .
It might take a few minutes for it to become available.