Deprecate Conditions CRD

Use `WhenExpressions` instead

The [API compatibility policy](https://github.com/tektoncd/pipeline/blob/master/api_compatibility_policy.md#alpha-beta-and-ga)
states that Alpha features may be dropped at any time, though users will
be given at least one release worth of warning.So we're giving two
releases warning on deprecation.

PR: tektoncd#3135
TEP: https://github.com/tektoncd/community/blob/master/teps/0007-conditions-beta.md

docs/deprecations.md

@@ -25,3 +25,4 @@ being deprecated.
 | [Step `workingDir` defaults to `/workspace`](https://github.com/tektoncd/pipeline/issues/1836)                                                | [v0.11.0-rc1](https://github.com/tektoncd/pipeline/releases/tag/v0.11.0-rc1) | Beta                                                                                                     | December 4 2020                     |
 | [The `TaskRun.Status.ResourceResults.ResourceRef` field is deprecated and will be removed.](https://github.com/tektoncd/pipeline/issues/2694) | [v0.14.0](https://github.com/tektoncd/pipeline/releases/tag/v0.14.0)         | Beta                                                                                                     | April 30 2021                       |
 | [The `PipelineRun.Spec.ServiceAccountNames` field is deprecated and will be removed.](https://github.com/tektoncd/pipeline/issues/2614)       | [v0.15.0](https://github.com/tektoncd/pipeline/releases/tag/v0.15.0)         | Beta                                                                                                     | May 15 2021                         |
+| [`Conditions` CRD is deprecated and will be removed. Use `WhenExpressions` instead.](https://github.com/tektoncd/community/blob/master/teps/0007-conditions-beta.md)       | [v0.16.0](https://github.com/tektoncd/pipeline/releases/tag/v0.16.0)         | Alpha                                                                                                     | Nov 02 2020                         |