Update Tekton version to 0.50.1, and Tekton API from v1beta1 to v1

[michaelsauter]

Closes #731.

This change unlocks, amongst other things, setting environment variables for tasks like this in ods.yaml:

pipelines:
- triggers: { .... }
  tasks:
  - name: build
    taskRef: { ... }
  taskRunSpecs:
  - pipelineTaskName: build
    podTemplate:
      env:
      - { name: MY_VAR, value: 'my value' }
    computeResources:
      requests:
        memory: 2Gi

Tasks:

• [x] Updated design documents in docs/design directory or not applicable
• [x] Updated user-facing documentation in docs directory or not applicable
• [x] Ran tests (e.g. make test) or not applicable
• [x] Updated changelog or not applicable