Fix usage graph not exceeding 100%

What does this MR do?

There was a bug report for the usage graph that it exceeded 100% and therefore caused an undesired behaviour: https://gitlab.com/gitlab-org/gitlab/-/issues/223030#note_370302384

This was caused by using toFixed which is unfortunately rounding the number. To prevent rounding errors, we are no longer rounding or truncating the ratio. We now also use flex instead of width.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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Availability and Testing

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• Tested in all supported browsers
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Security

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