compat: fix err msg in compat.<option>:is_new()

In the original commit 5f6d367c ("compat: add is_new and
is_old to options") `compat.<option_name>:is_new()` and `:is_old()`
were introduced, but by mistake they contained different usage
messages. This patch updates `:is_new()` usage msg to more
informative one from `:is_old()`.

Follows up #8807

NO_CHANGELOG=changelog from 5f6d367c is valid
NO_DOC=doc from 5f6d367c is valid