Ensure case contact form selects klanten backend from API group #9117

Job	Run time
Run the Django test suite (sequential)	4m 2s
Run the Django test suite (openklant)	3m 9s
End-to-end tests, firefox	4m 28s
Run the Django test suite (elastic)	5m 24s
Run the Django test suite (main)	23m 57s
Build and check documentation	1m 31s
End-to-end tests, webkit	5m 51s
End-to-end tests, msedge	5m 27s
End-to-end tests, chromium	3m 59s
Upload coverage information to codecov	17s
Build (and push) Docker image	1m 49s
	59m 54s

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