Promote SLSAv1.0 offering of Chains format to `beta`

This was discussed at the Chains WG on July 18th.
Since SLSAv1.0 was released, Chains provided support for it starting with provenance format slsa/v2alpha2. At this point, it is slsa/v2alpha4 (which includes support for StepActions and StepResults).

We propose stabilizing this to a Beta version: v2beta1 so that there is reduced immediate breaking changes and our users can adopt it more.

Here, I highlight some missing items:

• Implement support for Tekton Artifacts. The final PR is under review and we can introduce support for it in Chains right after.
• Identify the name of the pipelineTask/pipelineRun/taskRun/Step using the remote resource (Pipeline, Task, StepAction) in the resolved dependencies and byProducts. This will help with completeness.
• Capture the origin of a remote StepAction (i.e. uri and digest) in the resolved dependencies: https://github.com/tektoncd/pipeline/pull/8106

Start by reviewing Chains' current SLSA v2alpha4 provenance support and the final Tekton Artifacts PR referenced in the issue. Work through the three listed gaps—Tekton Artifacts, remote resource names, and StepAction origin—and define the compatibility and implementation scope for a v2beta1 offering. Done means the proposed beta support and these missing capabilities are addressed.