Bump com.google.cloud.tools:jib-core from 0.27.3 to 0.28.1 in /core

Bumps [com.google.cloud.tools:jib-core](https://github.com/GoogleContainerTools/jib) from 0.27.3 to 0.28.1.
- [Release notes](https://github.com/GoogleContainerTools/jib/releases)
- [Commits](https://github.com/GoogleContainerTools/jib/commits)

---
updated-dependencies:
- dependency-name: com.google.cloud.tools:jib-core
  dependency-version: 0.28.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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