Use workspace lints to control unexpected_cfgs.

[waywardmonkeys]

By using workspace.lints, we can configure lints once for everything (and then inherit it into the various crates).

The lint configuration for unexpected_cfgs works in 1.80+ and warns otherwise.

[waywardmonkeys]

I've marked this as a draft as there are unexpected cfgs still that need to be resolved:

• log in src_testbed (testbed: Remove use of missing log feature #684)
• other-backends isn't configured for rapier_testbed3d-f64 while it is for the other 3 configurations.

This will generate a warning from cargo in Rust 1.79 and earlier (1.80 is due for release soon) and there are already 2 warnings from cargo that are more verbose than this one.

I think this replaces #638 from @Urgau

To answer one of the concerns from @sebcrozet , advantages of this PR is that we get the benefit of the unexpected_cfgs lint again and we can start to use workspace.lints to centralize some other lint configurations in the future.

[waywardmonkeys]

Now that #684 has landed, I've dealt with the other-backends question in here and this should be ready now.

I've confirmed that it builds without warnings on beta builds (warnings from unexpected_cfgs).

[Vrixyz]

Thanks!

[waywardmonkeys]

You're welcome!