Move MSRV tests into the lint specific test files
There are currently two ways MSRV tests are done in the ui test suite, adding a case to the `#![clippy::msrv = "1.0"]` `tests/ui/min_rust_version_attr.rs` or adding the two `msrv_1_xx` functions to the test file of the lint in question
This updates the clippy book to suggest the `msrv_1_xx` style, and replaces the tests in `tests/ui/min_rust_version_attr.rs` with ones of that style
Almost the entire diff is just moving stuff around as a result, I made sure to check the line numbers the lints are emitted at correspond with the right `msrv` case, so feel free to only skim that part
changelog: none
28cd1ec9b7