I don't believe it's possible to perform a binding redirect to an assembly with a different public key.
You'll need to download the version of log4net v1.2.12.0 that is signed with the old key (http://psg.mtu.edu/pub/apache//logging/log4net/binaries/log4net-1.2.12-bin-oldkey.zip)
Then update your code, replacing references to:
log4net, Version=1.2.12.0, Culture=neutral, PublicKeyToken=669e0ddf0bb1aa2a
with
log4net, Version=1.2.12.0, Culture=neutral, PublicKeyToken=1b44e1d426115821
On a related note: you can put a version range in the redirect oldVersion="0.0.0.0-1.2.11.0"
, for added flexibility.