Auto merge of #7813 - xFrednet:6492-lint-version, r=flip1995

Add Clippy version to Clippy's lint list

Hey, hey, the semester is finally over, and I wanted to get back into hacking on Clippy. It has also been some time since our metadata collection monster has been feed. So, this PR adds a new attribute `clippy::version` to document which version a lint was stabilized. I considered using `git blame` but that would be very hacky and probably not accurate.

I'm also thinking that this attribute can be used to have a `clippy::nightly` lint group which is allow-by-default that delays setting the actual lint group until the defined version is reached. Just something to consider regarding #6623 :upside_down_face:

This PR only adds the version to 4 lints to keep it reviewable. I'll do a followup PR to add the version to other lints if the implementation is accepted :upside_down_face:

![image](https://user-images.githubusercontent.com/17087237/137118859-0aafdfdf-7595-4289-8ba4-33d58eb6991d.png)

Also, mobile approved xD

![image](https://user-images.githubusercontent.com/17087237/137118944-833cf7fb-a4a1-45d6-9af8-32c951822360.png)

---

r? `@flip1995`

cc: #7172

closes: #6492

changelog: [Clippy's lint list](https://rust-lang.github.io/rust-clippy/master/index.html) now displays the version a lint was added. :tada:

---

Example lint declaration after this update:

```rs
declare_clippy_lint! {
    /// [...]
    ///
    /// ### Example
    /// ```rust
    /// // Bad
    /// let x = 3.14;
    /// // Good
    /// let x = std::f32::consts::PI;
    /// ```
    #[clippy::version = "pre 1.29.0"]
    pub APPROX_CONSTANT,
    correctness,
    "the approximate of a known float constant (in `std::fXX::consts`)"
}
```

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