Support both v1alpha1 and v1beta1 for Custom Task

Support both v1alpha1.Run and v1beta1.CustomRun for 4 LTS releases
after Beta release so that custom task end users and developers are
able to fully test and migrate Custom Task from alpha to beta

To achieve this, this commit introduces a new feature flag to control
the version switch between alpha and beta when defining a custom task
within a Pipeline.

Author: XinruZhang

teps/0114-custom-tasks-beta.md

@@ -2,7 +2,7 @@
 status: implementable
 title: Custom Tasks Beta
 creation-date: '2022-07-12'
-last-updated: '2022-11-14'
+last-updated: '2022-11-24'
 authors:
 - '@jerop'
 - '@XinruZhang'
@@ -29,6 +29,7 @@ see-also:
       - [References and Specifications](#references-and-specifications)
       - [Remove Pod Template](#remove-pod-template)
       - [Feature Gates](#feature-gates)
+      - [Cancellation](#cancellation)
     - [Documentation](#documentation)
     - [Testing](#testing)
   - [Optional](#optional)
@@ -205,12 +206,28 @@ own specification.
 
 ##### Feature Gates
 
+##### New Feature Flag `custom-task-version`
+
+Support both `v1alpha1.Run` and `v1beta1.CustomRun` for **four** [long term support (LTS)][long-term-support] 
+releases. So that custom task implementors and end users are able to fully test and migrate Custom Task from
+`v1alpha1` to `v1beta1`. 
+
+Create a new feature flag `custom-task-version` to allow end users to choose which version of custom task to be 
+created out of a `PipelineTask`. We set the default feature flag value to `v1alpha1` in the first LTS release, and
+switch to `v1beta1` in the following three LTS releases.
+
+Say we start to support `v1beta1.CustomRun` in LTS release `X`, and set default value of `custom-task-version`
+to `v1alpha1`. In the LTS release `X+1`, we switch the default value to `v1beta1`. In the LTS release `X+4`, we
+remove the feature flag, and only support `v1beta1`.
+
+##### Existing Feature Gates
+
 Remove guarding of `Custom Tasks` behind `enable-custom-tasks` and `enable-api-fields` feature gates.
 
 When [TEP-0096: Pipelines V1 API][tep-0096] is implemented to add V1 API, `Custom Tasks` will be gated behind feature
 gate `enable-api-fields` being set to `"beta"` - this is out of scope for this TEP  (in scope for TEP-0096).
 
-##### Cancellation
+#### Cancellation
 
 The custom task is responsible for implementing `cancellation` to support pipelineRun level `timeouts` and `cancellation`. If the Custom Task implementor does not support cancellation via `.spec.status`, `Pipeline` **can not** timeout within the specified interval/duration and **can not** be cancelled as expected upon request.
 
@@ -366,3 +383,4 @@ For further details, see [tektoncd/community#523][523], [tektoncd/community#667]
 [5138]: https://github.com/tektoncd/pipeline/issues/5138
 [cancel-pr]: https://github.com/tektoncd/pipeline/blob/main/docs/pipelineruns.md#cancelling-a-pipelinerun
 [gracefully-cancel-pr]: (https://github.com/tektoncd/pipeline/blob/main/docs/pipelineruns.md#gracefully-cancelling-a-pipelinerun)
+[long-term-support]: https://github.com/tektoncd/community/blob/main/releases.md#support-policy

teps/README.md

@@ -281,7 +281,7 @@ This is the complete list of Tekton teps:
 |[TEP-0110](0110-decouple-catalog-organization-and-reference.md) | Decouple Catalog Organization and Resource Reference | implemented | 2022-06-29 |
 |[TEP-0111](0111-propagating-workspaces.md) | Propagating Workspaces | implemented | 2022-09-16 |
 |[TEP-0112](0112-replace-volumes-with-workspaces.md) | Replace Volumes with Workspaces | proposed | 2022-07-20 |
-|[TEP-0114](0114-custom-tasks-beta.md) | Custom Tasks Beta | implementable | 2022-11-14 |
+|[TEP-0114](0114-custom-tasks-beta.md) | Custom Tasks Beta | implementable | 2022-11-24 |
 |[TEP-0115](0115-tekton-catalog-git-based-versioning.md) | Tekton Catalog Git-Based Versioning | implementable | 2022-11-09 |
 |[TEP-0116](0116-referencing-finally-task-results-in-pipeline-results.md) | Referencing Finally Task Results in Pipeline Results | implemented | 2022-08-11 |
 |[TEP-0117](0117-tekton-results-logs.md) | Tekton Results Logs | implementable | 2022-10-21 |