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Purpose of this PR

There's a drive for better Input System adoption in 6000.1 and one way we're doing this is by updating key templates. All this PR does is change the input backend to Input System exclusive and adds a dependency for it.

Testing status

Ran every available scene looking for unexpected issues. The third person controller in the hd template already uses Input System so that worked fine out of the box

Comments to reviewers

Do note that this is going into the master branch. I don't know how to target the 6.1 Editor only 😅 I believe the graphics team will handle landing though