pom.xml: updates for release 0.51.19
pom.xml: updates for release 0.51.19

Tweak ErrorCode message for TAG_DEFINITION_XX and remove unused ObjectType.REFUND

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

killbill/killbill-api,killbill/killbill-api