fix(lint): Warn not Error on unsupported lint tool

[Muscraft]

In a recent Cargo Team meeting, it was decided to lessen the error on an unsupported tool in [lints] to a warning. This PR implements that change.

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[Muscraft]

I also considered:

`cargo` reserves all of the `lints` namespace for itself and provides no
future compatibility guarantees when encountering unrecognized tools, as they
may be supported in the future.

[weihanglo]

I prefer this one.

Should we also suggest using package.metadata?

[epage]

I feel like this makes it sound like the lints table is special and this doesn't apply elsewhere.

Something to keep in mind is people might also be here from a typo.

[Muscraft]

I think this should be the top level message:

unrecognized lint tool `{}`, must be one of {supported}

What about:

no compatibility guarantees are provided when specifying unrecognized manifest
key(s), as the entire namespace is reserved for future use. Tools that wish to
store package configuration should use `package.metadata`

or

no compatibility guarantees are provided when specifying unrecognized manifest
key(s) as `cargo` reserves the entire namespace for future use. `package.metadata`
should be used for tools which would like to store package configuration

[epage]

I'm good with the now updated wording

[epage]

@bors r+

[bors]

📌 Commit 72f265b has been approved by epage

It is now in the queue for this repository.

[bors]

⌛ Testing commit 72f265b with merge 1c92c1e...

[bors]

☀️ Test successful - checks-actions
Approved by: epage
Pushing 1c92c1e to master...