time-stamp: revert recent change to "%04y"

time-stamp: revert recent change to "%04y"

* time-stamp.el (time-stamp-string-preprocess): Revert change to "%04y"
format made 2 weeks ago by commit 0e56883878 (the previous commit to
this file).  Although undocumented, "%04y" was discovered to be in use
in the wild (2016) and had not issued a warning that it would change.
Add a warning that it will change.

* time-stamp-tests.el (time-stamp-test-year-2digit): add test of "%04y"