Refine and possibly deprecate whether or not plural translations should support float/non-integer values

Problem/Motivation

#2660338: Deprecate locale_get_plural make support for float values kind of explicit, which was needed for BC, as you could possibly do that before as well (although with PHP deprecations on more recent PHP versions on implicit float to integer conversion).

It's unclear what the behavior for float values should be, if they should be rounded or what that even means. It kind of works for languages with a single plural version as anything not exactly 1 (including 1.0) uses the plural version, so .6 hours and 1.3 hours, but I don't know any languages with multiple plural versions, and what for example 2.5 should use when you have a rule for 2 and 3.

Steps to reproduce

Proposed resolution

Remaining tasks

Figure out whether supporting floats make sense, possibly adjust the implementation if yes, deprecate it if not.

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet