fixing Volto delivers HTTP 200 when no backend server is available

[Shyam-Raghuwanshi]

Now we are returning a 503 status code if the backend is not running

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• I successfully ran code linting checks on my changes locally.
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Closes #6447

[aryan7081]

@Shyam-Raghuwanshi Please add a Change log entry: https://6.docs.plone.org/volto/contributing/index.html#change-log-entry

[Shyam-Raghuwanshi]

6447

yes i forgot