Confluent dependencies are referenced via http instead of https

[spanglerco]

When the pom.xml file references the confluent maven repository, it uses http://packages.confluent.io/maven/ (insecure) instead of https://packages.confluent.io/maven/ (secure).

This conflicts with the 3.8.1+ versions of maven, which block the use of HTTP repositories by default for security reasons. Building this repository with the latest maven fails for this reason.