release schedule: add April targets for 1.18.1 and 1.18.2
release schedule: add April targets for 1.18.1 and 1.18.2

We're on track for a set of .1 bugfixes for 1.18, and then aim to have
that branch sync into the monthly patch release cadence with the other
active branches.

Signed-off-by: Tim Pepper <4807564e5a18784afafcf20dbdb5591e844b9075@vmware.com>

kubernetes/sig-release