Add propagated labels to API Compatibility Policy

During an API Working Group call on Mon Apr 20th folks generally
agreed that the labels propagated from PipelineRuns to TaskRuns
and TaskRuns to Pods should be considered part of the API Compatibility
Policy.

These labels include:

tekton.dev/task=<Task Name>
tekton.dev/taskRun=<TaskRun Name>
tekton.dev/pipeline=<Pipeline Name>
tekton.dev/pipelineRun=<PipelineRun Name>
tekton.dev/pipelineTask=<Pipeline Task Name>
tekton.dev/conditionCheck=<Condition Check Name>

These labels may already be relied upon by teams and platforms as the
means to identify resources created during Run execution.

This PR adds the labels to the list of items that are considered
covered by the API compatibility policy.

api_compatibility_policy.md

@@ -16,6 +16,7 @@ The API is considered to consist of:
 - The structure of the CRDs including the `spec` and `status` sections, as well as:
  - The ordering of the `steps` within the `status`
  - The naming of the `step` containers within the `status`
+ - The labels propagated from `PipelineRuns` to `TaskRuns` and `TaskRuns` to `Pods`.
 - The structure of the [directories created in executing containers by Tekton](docs/tasks.md#reserved-directories)
 - The order that `PipelineResources` declared within a `Task` are applied in
 - The interfaces of the images that are built as part of Tekton Pipelines,
@@ -76,4 +77,4 @@ spec. These changes will mean that folks using a previous version of the API wil
 to adjust their usage in order to use the new version.
 
 These changes must be approved by [more than half of the project OWNERS](OWNERS)
-(i.e. 50% + 1).
+(i.e. 50% + 1).