Refactor: config option parent -> parents

[rarkins]

Describe the proposed change(s).

Change this to be an array of strings:

renovate/lib/config/types.ts

Lines 397 to 402 in 8cf2d21

	parent?:
	| 'customDatasources'
	| 'hostRules'
	| 'packageRules'
	| 'postUpgradeTasks'
	| 'customManagers';

• associated downstream code changes