…ocritic)

… check for specific errors (errorlint)

Bump module name to `v2`, because it has incompatible changes.

Args of `NewThresholds()` must be type of `cmp.Ordered` and it can't be `nil`.

Two new methods added to `Thresholds` type:

```
UseWarning(useMin, useMax bool)
UseCritical(useMin, useMax bool)
```

`useMin` defines is `WarningMin` or `CriticalMin` defined or not. `useMax` does
the same for `WarningMax` or `CriticalMax`. So instead of calling
`NewThresholds()` with `nil`s, now we should use `UseWarning()` and/or
`UseCritical()`.

There is no `SetWarn()` and `SetCrit()` methods. We use `SetThresholds()` and
`NewThresholds()` instead.

because it automatically uses same type as Value.

which allows replace the default OK message at any time later.