♻️ Cleanup diffIn* diffInReal* floatDiffIn* floatDiffInReal*

[kylekatarnls]

📅 Historically, Carbon had diffIn* methods returning integer values relying on ->diff() PHP native which is sensitive to DST.

Then it appears that a floating result was more convenient. And despite the integer result can easily be get from the float result, the existing methods (for each unit) has been kept for backward compatibility and an other set has been implemented for "float" result.

Then 2 other sets have been added to work around the native PHP DST bugs and use the timestamp instead. So the "Real" methods appeared. Using GMT for calculation (which is what we promote), it makes no difference. Old methods have just been kept for backward compatibility for those who run calculations with a timezone setting having DST.

🚀 For the version 3.0 (branch: 3.x) we need to get rid of the old methods. We will just provide 1 set of diffIn* method (only 1 for each unit) which will return a float using timestamps (ignoring DST).

Users wanting the old DST-behavior would have to code their own as we discourage this approach.

Users wanting integer result will have to explicitly use (int), floor(), ceil() or round() which is actually a good thing. We actually use (int) which truncate the result (equivalent to floor() for positive numbers, but to ceil() for negative numbers) and this is actually arbitrary. I think it will give a better control to users to always give a result as precise as possible and let him decide how to reduce precision on need.

If someone want to take this task, pull-requests on 3.x branch are welcome. :D