gh-37995: Remove interactive confirmation for installing experimental packages
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This removes some complexity and eliminates the pitfall that Sage is
waiting for confirmation from the user but the query is hidden in long
terminal output from other package installations.
With this PR we just exit with an error and tell users what to do.
Fixes #35670.
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/37995
Reported by: Matthias Köppe
Reviewer(s):
