Use cloud-events-bom for io.cloudevents.xml

[cppwfs]

Currently we manage the version for io.cloudevents.xml. This is because it was omitted from the bom provided by cloudevents. This is being tracked by cloudevents/sdk-java#701. Once resolved, we can update our our build.gradle and toml file.

[artembilan]

Moved to Backlog since it is unlikely Cloud Events 4.1 will make it in time for our 7.1