Update step-runner version to 0.15.0

What does this MR do?

Update step-runner version to 0.15.0.

Changes

From the step-runner changelog:

• v0.15.0
  • Add version flag to step-runner binary. See !265.
  • The promote step image step accepts to_image input instead of to_registry, and to_repository. See !271.
  • File executable permissions are retained when building and fetching a step image. See !267.
• v0.14.0
  • The build OCI image step allows paths to be specified relative to the CI_PROJECT_DIR. See !255.
  • Authenticate when fetching from an OCI registry using the DOCKER_AUTH_CONFIG environment variable. See !256.
  • Build darwin step-runner images. See !259.
  • Expressions in struct keys are evaluated as strings. See !258.
  • The build step image step accepts image input instead of registry, repository and tag. See !260.
  • Rename build step output ref to image. See !260.
  • Build step requires semantic version. See !262.
  • Remove to-version input from promote step. See !264.