Change license_file to license_files in setup.cfg (#5948)

# Description

During installation from source, the following message is presented: 

```
********************************************************************************
The license_file parameter is deprecated, use license_files instead.

By 2023-Oct-30, you need to update your project and remove deprecated calls
or your builds will no longer be supported.

See https://setuptools.pypa.io/en/latest/userguide/declarative_config.html for details.
********************************************************************************
```

This PR changes the parameter name and bumps the minimum setuptools
version. Maybe it is possible to use lower version, but they do not put
such information anywhere.

# References

pypa/setuptools#2676

## Type of change
<!-- Please delete options that are not relevant. -->
- [x] Bug-fix (non-breaking change which fixes an issue)

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = [
-  "setuptools >= 42",
+  "setuptools >= 56",
   "wheel",
   "setuptools_scm[toml]>=3.4"
 ]

setup.cfg

@@ -3,7 +3,7 @@ name = napari
 url = https://napari.org
 download_url = https://github.com/napari/napari
 license = BSD 3-Clause
-license_file = LICENSE
+license_files = LICENSE
 description = n-dimensional array viewer in Python
 long_description = file: README.md
 long_description_content_type = text/markdown