Reflect PSF-2.0 being an official SPDX license

[xhochy]

See the official description that the project is licenses under the PSF license: https://matplotlib.org/stable/users/project/license.html As PSF-2.0 is now an official SPDX license identifier, we can drop the LicenseRef- prefix.

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [x] Reset the build number to 0 (if the version changed)
• [x] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [x] Ensured the license file is being packaged.

[conda-forge-webservices[bot]]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[xhochy]

@conda-forge/matplotlib This is ready for review/merge.