Rollup merge of #141559 - RalfJung:less-rustc_allow_const_fn_unstable, r=compiler-errors

const-check: stop recommending the use of rustc_allow_const_fn_unstable

I have seen way too many people see the compiler suggest this attribute and then just apply it without a second thought. This is bad. So let's just stop recommending it; for the rare case where someone needs it, they'll eventually ask us and that way we can be sure that it is truly needed. The dev-guide still also explains `rustc_allow_const_fn_unstable`.

Cc ``@rust-lang/wg-const-eval``

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