Rely on native icu for now.

[cbrueffer]

Checklist

• [x] Used a fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [ ] 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.

Update to depend on native icu. The r-base recipe already does this for all builds except win, so there's no reason not to rely on it.

See https://github.com/conda-forge/staged-recipes/issues/6690 for discussion.

[conda-forge-linter]

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.

I do have some suggestions for making it better though...

For recipe:

• Using toolchain directly in this manner is deprecated. Consider using the compilers outlined here.