fix(options)!: Make not_planned the default close-issue-reason (#807)

* docs: add source for default testing options

* fix(options)!: Default to 'Close as not planned'

GitHub introduced an additional close reason 'Close as not planned':
https://github.blog/changelog/2022-05-19-the-new-github-issues-may-19th-update/

'stale' is a use case for this close reason

Fixes #789

BREAKING CHANGE:
`close-issue-reason` defaults to `not_planned` (previously: ``
which defaulted to `completed`)
This closes the issue as 'not planned' on GitHub rather than 'completed'

See: https://github.blog/changelog/2022-05-19-the-new-github-issues-may-19th-update

__tests__/constants/default-processor-options.ts

@@ -1,5 +1,7 @@
 import {IIssuesProcessorOptions} from '../../src/interfaces/issues-processor-options';
 
+// Default options for use in tests.
+// Mirrors the defaults defined in action.yml
 export const DefaultProcessorOptions: IIssuesProcessorOptions = Object.freeze({
   repoToken: 'none',
   staleIssueMessage: 'This issue is stale',
@@ -51,6 +53,6 @@ export const DefaultProcessorOptions: IIssuesProcessorOptions = Object.freeze({
   ignoreIssueUpdates: undefined,
   ignorePrUpdates: undefined,
   exemptDraftPr: false,
-  closeIssueReason: '',
+  closeIssueReason: 'not_planned',
   includeOnlyAssigned: false
 });