og_image: Use binary prefixes for format_bytes

[T0mstone]

Usage of SI prefixes to denote powers of 1024 is incorrect:

[T]he major standards organizations have expressly disapproved the use of SI prefixes to denote binary multiples, and recommended or mandated the use of the IEC prefixes for that purpose[.]

This just changes them to use the proper kiB and MiB instead of kB and MB.

An alternative would be changing the code instead, to use powers of 1000 like format_number does, but that might be a bit ambiguous precisely because this error is still so common.

[T0mstone]

Oh, looking at the Wiki page again, I guess it should be Ki instead of ki.

[T0mstone]

I don't understand why the tests are failing in CI but passing on my machine.

[Turbo87]

I don't understand why the tests are failing in CI but passing on my machine.

Might have been related to #11530

I've rebased it and CI seems to be happy now