Minor: fixup contribution guide about clippy

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

While working on #5907 I ran into this ancient piece of advice that is not accurate anymore

What changes are included in this PR?

Update to contributing guide to reflect the current state with clippy

Are there any user-facing changes?

[Xuanwo]

This doesn't work on my archlinux x86_64:

error: sub-expression diverges
   --> arrow-flight/tests/flight_sql_client_cli.rs:657:9
    |
657 |         unimplemented!("Implement do_action_end_savepoint")
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#diverging_sub_expression
    = note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `unimplemented` (in Nightly builds, run with -Z macro-backtrace for more info)

error: sub-expression diverges
   --> arrow-flight/tests/flight_sql_client_cli.rs:665:9
    |
665 |         unimplemented!("Implement do_action_cancel_query")
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#diverging_sub_expression
    = note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `unimplemented` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `arrow-flight` (test "flight_sql_client_cli") due to 7 previous errors

[alamb]

That looks like it may be due to the fact you are using nightly rust by default

does it work with:

cargo +stable clippy --workspace --all-targets --all-features -- -D warnings