pom.xml: updates for release 0.73
pom.xml: updates for release 0.73

Update killbill-api.version -> 0.31

Signed-off-by: Kill Bill core team <1a73af9e7ae00182733b2292511b814be66f065f@killbill.io>

rocketlawyer/killbill-oss-parent,killbill/killbill-oss-parent