Deprecate Flex and FlexItem components in favor of BoxComponent about view_components

[srt32]

Primer Components currently say

The Flex component behaves the same as the Box component except that it has display: flex set by default.

Previously, a Flex.Item component was used for flex item specific properties; Box now contains all those properties and should be used in its place.

so, at minimum we could deprecate FlexItem to be consistent.

Assuming we're agreed on that, we are left to discuss if we want to keep FlexComponent.

cc/ @joelhawksley @manuelpuyol thoughts?

[joelhawksley]

@srt32 I think it might be worth looking at how we use these components internally and consider refactoring them to use Box. If that goes well, deprecation sounds like the right route.

[manuelpuyol]

I believe that flex properties could be part of Classify (system_arguments)