CodingGuidelines: give deadline for "for (int i = 0; ..."

CodingGuidelines: give deadline for "for (int i = 0; ..."

We raised the weather balloon to see if we can allow the construct
in 44ba10d6 (revision: use C99 declaration of variable in for()
loop, 2021-11-14), which was shipped as a part of Git v2.35.
Document that fact in the coding guidelines, and more importantly,
give ourselves a deadline to revisit and update.

Let's declare that we will officially adopt the variable declaration
in the initializaiton part of "for ()" statement this winter, unless
we find that a platform we care about does not grok it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>