Add an explicit deprecation date

So that we can remove support for the beta configuration of
`on: push`-only we should communicate a date after which this
will not work.

src/actions-util.ts

@@ -157,7 +157,7 @@ function toCodedErrors(errors: {
 export const WorkflowErrors = toCodedErrors({
   MismatchedBranches: `Please make sure that every branch in on.pull_request is also in on.push so that Code Scanning can compare pull requests against the state of the base branch.`,
   MissingHooks: `Please specify on.push and on.pull_request hooks so that Code Scanning can compare pull requests against the state of the base branch.`,
-  MissingPullRequestHook: `Please specify an on.pull_request hook so that Code Scanning is run against pull requests.`,
+  MissingPullRequestHook: `Please specify an on.pull_request hook so that Code Scanning is explicitly run against pull requests. This will be required to see results on pull requests from January 31 2021.`,
   MissingPushHook: `Please specify an on.push hook so that Code Scanning can compare pull requests against the state of the base branch.`,
   PathsSpecified: `Using on.push.paths can prevent Code Scanning annotating new alerts in your pull requests.`,
   PathsIgnoreSpecified: `Using on.push.paths-ignore can prevent Code Scanning annotating new alerts in your pull requests.`,