Correctly set the spring context-path for api-docs

[PierreBesson]

This is an alternative to #12923 that wanted to support monolith's frontend with a custom context-path as a minimal support for JHipster 7. So This will only work for the back-end part.

With @pascalgrimaud we realized that there are many places where we should set the context-path for the UI including cypress.json, webpack, and this cannot be set dynamically so it would need to be a generator option.

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