Update Go to 1.15 in Releases + Check Licenses CI

[runewake2]

This introduces a new CI Check that runs the create-licenses.sh script and saves the results for 7 days in a GitHub Artifact for reference. These artifacts can be viewed while available by browsing the GitHub Action job (Example available for next week: https://github.com/runewake2/kpt/actions/runs/709503762). This allows the resulting licenses file to be manually validated if needed. Currently a job failure should not produce an artifact. More info about how artifact uploads work is here.

Additionally Kpt's GitHub Actions and Docker Images used during Cloud Build have been brought into alignment on Go Version. They should all use Go version 1.15 now.

The create-licenses.sh script has been updated to support 1.15.

The new CI check is included to help verify these changes and so licensing issues are detected during the PR's that would introduce them.

Note: This should be merged into kpt next as well. While the workflow file mentions next this is only to keep the workflow definitions identical across branches. This PR should not cause pr's or pushes against next to run this job until it is merged into the next branch.