docs/UPGRADING.md: Add a removal date for the old row/col based indexing

While we added the new APIs and the entry in UPGRADING.md last summer, a removal
date was never announced. Lets fix that now, and remove these in March.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

docs/UPGRADING.md

@@ -495,7 +495,7 @@ After renaming, some of the original symbols have been deprecated. The deprecate
 Row/col based indexing was replaced by linear indexing throughout the whole firmware. A compatibility layer of functions was introduced that allows
 the firmware to remain backwards compatible, however, these functions are deprecated and will be removed in future versions of the firmware.
 
-Also a new version of the onKeyswitchEvent-handler has been introduced. The old version is deprecated.
+Also a new version of the onKeyswitchEvent-handler has been introduced. The old version is deprecated, and will be removed after **2020-03-15**.
 
 ### Source code and namespace rearrangement