Replace `gl-flex-wrap` by `gl-flex-wrap-wrap`

What does this MR do and why?

Replace gl-flex-wrap by gl-flex-wrap-wrap

The gl-flex-wrap class is deprecated in favor of gl-flex-wrap-wrap. This is more consistent with our name convention of gl-{breakpoint?}-{property}-{value}.

Relates to gitlab-ui#1951 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

I tested locally using the billing page following these steps: !83774 (merged)

MR acceptance checklist

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