The CNB specifications allow builders to support multiple platform
API versions. The supported versions are published in the builder
image metadata as an array of version numbers, while a single
supported version number was published in earlier builder metadata.

These changes read the supported versions from the builder metadata
and fall back to the single version if the array is not present.
A CNB_PLATFORM_API environment variable is set on each lifecycle
phase invocation to request a specific version as recommended in
the CNB platform spec.

Fixes gh-23682