admin: change nikerabbit's key

Nik asked this as a security measure given the compromise of
translatewiki. We have no evidence of any compromise and this is
just a cautionary measure.

RT #7834

Change-Id: I05e9c0943cce941a6ba4866b4f62b3d634484c75
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>