After removing the string size from `VariantContent`, `deserializeJson()` and `deserializeMsgPack()` could not support NUL in strings in the zero-copy mode anymore.
Instead of adding a complicated warning in the documentation, I thought it was better to remove the zero-copy mode entirely.
The zero-copy mode has always been a source of bugs because many users used it without realizing it.
Also, the memory savings are smaller now that we deduplicate strings, so this feature should not be missed much.