Change while_let_on_iterator suggestion to use by_ref()

[Jarcho]

It came up in the discussion #7659 that suggesting iter.by_ref() is a clearer suggestion than &mut iter. I personally think they're equivalent, but if by_ref() is clearer to people then that should be the suggestion.

changelog: Change while_let_on_iterator suggestion when using &mut to use by_ref()

[rust-highfive]

r? @xFrednet

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[xFrednet]

Thank you for the PR.

I can see the reasoning why by_ref() might be preferred, even if they are equivalent. Using by_ref() looks a bit cleaner IMO, and it's also easier to find documentation for it. The std docs contains an example which illustrates the usage of by_ref() quite well. 🙃

One related question: It seems like this resolves the discussion from #7659 should we close the issue after this got merged? (I'll r+ this PR now, as it looks good to me. We can close the issue manually afterwards)

[xFrednet]

@bors r+

[bors]

📌 Commit ee6a6b5 has been approved by xFrednet

[bors]

⌛ Testing commit ee6a6b5 with merge 871ad80...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing 871ad80 to master...

[Jarcho]

There's still the question of whether the lint should be split between the two cases (whether by_ref() is needed or not).