Help: Remove wrong versionadded for CMAKE_COMPILER_IS_GNU* variables and add a deprecation notice
!4947 (merged) added the versionadded directive to many variables, but it wrongly detected the commit a6d3f541 (Help: Clarify documentation of CMAKE_COMPILER_IS_GNU{CC,CXX,G77}, 2016-09-14, v3.7.0-rc1~120^2~1) as the origin for these variables. In reality, these variables were introduced in commit f5d95fb0 (Complete rework of makefile generators expect trouble, 2002-11-08, v2.4.0~4935).
Also adds a deprecation notice without runtime warnings to the docs.
Fixes: #23526 (closed)