[mixcloud] updated mixcloud API server address

[kmnx]

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Description of your pull request and other information

The mixcloud API address changed from www.mixcloud.com/graphql to app.mixcloud.com/graphql
This PR updates the address to restore functionality.

[dirkf]

Is there an issue for this problem, or are #32383, #32387 related?

[kmnx]

there is no issue for this PR.
the mentioned issues are unrelated, both issues are from before the API URL changed.
also one is about soundcloud, not mixcloud.
i can't reproduce either issue as soundcloud works instantly and mixcloud works with this PR. even without this PR the error is different, mixcloud does not give a 403 for me even with wrong API URL.