Bump itk-core from 5.4rc3 to 6.0a1 in /envs

Bumps [itk-core](https://github.com/InsightSoftwareConsortium/ITK) from 5.4rc3 to 6.0a1.
- [Release notes](https://github.com/InsightSoftwareConsortium/ITK/releases)
- [Commits](https://github.com/InsightSoftwareConsortium/ITK/commits)

---
updated-dependencies:
- dependency-name: itk-core
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

envs/requirements.txt

@@ -13,7 +13,7 @@ imageio==2.34.0
 importlib_metadata==7.1.0
 importlib_resources==6.4.0
 itk==5.4rc3
-itk-core==5.4rc3
+itk-core==6.0a1
 itk-elastix==0.19.2
 itk-filtering==5.4rc3
 itk-io==5.4rc3