Make warning about disabling PyPI become true and deprecate default source

[radoering]

Pull Request Check List

Continues #7801 (see target image in issue description)

• Added tests for changed code.
• Updated documentation for changed code.

If there is a primary source, no default source and PyPI is not configured explicitly, we print the following warning since Poetry 1.5:

"Warning: In a future version of Poetry, PyPI will be disabled"
" automatically if at least one custom primary source is"
" configured. In order to avoid"
" a breaking change and make your pyproject.toml forward"
" compatible, add PyPI explicitly via 'poetry source add pypi'."
" By the way, this has the advantage that you can set the"
" priority of PyPI as with any other source."

Since we are printing it for three versions now, I suppose we can consider to make this warning become true. When we do that, the priority default is the same as the topmost primary source (if there is no default source). Thus, we can deprecate default to make source priorities simpler.

[github-actions]

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