Fix icons displayed in BCD cells

[queengooborg]

This PR fixes the rendering of the prefix, altname and flags icons on BCD cells to ensure they aren't unintentionally shown when not applicable. This should help reduce issues and confusion such as mdn/browser-compat-data#17208.

[danielhjacobs]

The reason for the switch in logic was pages like https://developer.mozilla.org/en-US/docs/Web/CSS/flex-flow#browser_compatibility. This will go back to showing Firefox as needing a vendor prefix, since support for a vendor prefix was added after support without a vendor prefix. That's why my proposed fix was #6663

See also #5721

[queengooborg]

This will go back to showing Firefox as needing a vendor prefix, since support for a vendor prefix was added after support without a vendor prefix.

Actually, this PR's intent is to do the opposite, and NOT show needing a vendor prefix when it shouldn't. On https://developer.mozilla.org/en-US/docs/Web/CSS/transition#browser_compatibility, such an issue can be found. It seems that #6663 fixes a good chunk of the problems, but there's still one or two that can be found.

One of the benefits of this approach is that it will always match the result from the getCurrentSupport() function, the same function used to select what version to display in the cell. I haven't seen any issues so far with the function, so I've trusted it to make the right call on pages.

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To help show the changes this PR makes, this is the transition CSS property page, before and after:

And to show that other pages like flex-flow are unaffected:

[danielhjacobs]

Oh right, I forgot I added a ranking system in #5946 to that function. This makes sense then, yeah, Using it wasn't an option before since I hadn't changed it yet.

In that case, my feedback is this: Would this work the same if you remove those three functions and switch back to the pre- #5721 statements?

[danielhjacobs]

In other words, just supportItem.prefix, supportItem.alternative_name, and supportItem.flags

[queengooborg]

Sure thing, changes coming right up!

[danielhjacobs]

Note: Also fixes mdn/browser-compat-data#16839 and fixes mdn/browser-compat-data#16833

[fiji-flo]

Thanks for fixing!