Read and use deprecated configuration (as well as emitting a warning)
Original change written by `@flip1995` I've simply rebased to master and fixed up the formatting/tests. This change teaches the configuration parser which config key replaced a deprecated key and attempts to populate the latter from the former. If both keys are provided this fails with a duplicate key error (rather than attempting to guess which the user intended).
Currently this on affects `cyclomatic-complexity-threshold` -> `cognitive-complexity-threshold` but will also be used in #8974 to handle `blacklisted-names` -> `disallowed-names`.
```
changelog: deprecated configuration keys are still applied as if they were provided as their non-deprecated name.
```
- [x] `cargo test` passes locally
- [x] Run `cargo dev fmt`
53a09d4855