scheduled_task: Remove use of stub (Rspec 2.0) from tests

The stub command is considered old and shouldn't be used as per
https://relishapp.com/rspec/rspec-mocks/v/3-7/docs/old-syntax/stub

e.g. in
https://github.com/puppetlabs/puppetlabs-scheduled_task/blob/b06f6ef19fd4094738ccf61cc29b36b05a6b9111/spec/unit/puppet/provider/scheduled_task/win32_taskscheduler_spec.rb#L145-L147

This ticket will track the work to remove the usage of older rspec things and use Rspec 3.x equivalents.