fix(stepper): avoid breaking change in stepControl type

[crisbeto]

In #15134 we reworked the stepper not to depend on @angular/forms under the assumption that our limited FormControl interface would be enough to avoid a breaking change. Some people ended up being broken by the change so this PR reworks the stepControl type to avoid the breaking change.

Fixes #15462.

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
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     status "ci/circleci: tests_browserstack" is pending
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[biltongza]

This is still a breaking change for us since FormControlLike is not exported. We depended on this being AbstractControl in one of our methods to mark all child controls as touched, and now the only option we really have is to cast the FormControlLike to AbstractControl. Is that the recommended option?

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