Ensure indices are not used as React keys

[sergei-maertens]

For example: https://github.com/open-formulieren/formio-renderer/pull/4/files#diff-9185d63b2c63822bf756d0c5012592387413b1089b5ea674d96d922e159e4e15R74

This causes problems when lists are being mutated - e.g. repeating groups (edit grid) with 3 items where item at index 0 or 1 is deleted -> react won't re-render the old item 0 and 1.

This should be properly asserted with user interaction tests in storybook.