refactor: avoid exposing details like `copy` in the builder APIs

[cpcloud]

What happened?

From @NickCrews in #6768:

Unrelated to this, but I noticed that the exposed API of the CaseBuilders is probably a little more than we want. We probably just want people to have access to .when(), .else_(), and .end(), but since CaseBuilder inherits from Concrete, users get type hint suggestions for .copy(), and the other Expr internals. But that is separate from this issue

What version of ibis are you using?

master

What backend(s) are you using, if any?

N/A

Relevant log output

No response

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[cpcloud]

If we move to cases then this shouldn't be an issue anymore.