pom.xml: updates for release 0.21.3
pom.xml: updates for release 0.21.3

Update platform dependency to fix usage plugin api

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

sbrossie/killbill,killbill/killbill,killbill/killbill,sbrossie/killbill,killbill/killbill,sbrossie/killbill,sbrossie/killbill,killbill/killbill,killbill/killbill,sbrossie/killbill