Don't warn when both license and license-file are specified

[brettcannon]

Describe the problem you are trying to solve

Right now cargo warns if you define both license and license-file:

cargo/src/cargo/util/toml/mod.rs

Lines 1439 to 1445 in 72ef2bd

	if project.license_file.is_some() && project.license.is_some() {
	manifest.warnings_mut().add_warning(
	"only one of `license` or \
	`license-file` is necessary"
	.to_string(),
	);
	}

This is a bit unfortunate as it means any tools that try to collect license files (e.g. for a 3rd-party notices file) have to write their own logic to try and figure out where the license is. But if projects could specify both the SPDX expression in license to know if a project meets any licensing requirements while also specifying license-file so it's easy to collect the license text as well would be helpful.

Describe the solution you'd like

1. No longer warn when both license and license-file are specified.
2. Update the docs to state that at least_ one of the two fields should be specified, but specifying both is encouraged.

[Eh2406]

There is some previous discussion at #8537

[brettcannon]

Based on the discussion that @Eh2406 linked to, I think this will eventually get rejected, so I'm going to preemptively close this.