Rollup merge of #77136 - ecstatic-morse:issue-77134, r=oli-obk

Suggest `const_mut_refs`, not `const_fn` for mutable references in `const fn`

Resolves #77134.

Prior to #76850, most uses of `&mut` in `const fn` ~~required~~ involved two feature gates, `const_mut_refs` and `const_fn`. The first allowed all mutable borrows of locals. The second allowed only locals, arguments and return values whose types contained `&mut`. I switched the second check to the `const_mut_refs` gate. However, I forgot update the error message with the new suggestion.

Alternatively, we could revert to having two different feature gates for this. OP's code never borrows anything mutably, so it didn't need `const_mut_refs` in the past, only `const_fn`. I'd prefer to keep everything under a single gate, however.

r? @oli-obk

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