Update Jenkinsfile to use docker-debian-jdk8

[cchristous]

Problem

The existing Jenkins job fails to download the latest version of common-tools, which is needed for some standard Jenkins job logic.

Solution

Use the docker-debian-jdk8 node type instead of the existing one. This node type is as similar to the existing node type as possible and successfully downloads the latest version of common-tools. The same approach was taken on a similar repo a couple years ago: https://github.com/confluentinc/kafka-connect-sqs/pull/30

Does this solution apply anywhere else?

• [x] yes
• [ ] no

If yes, where?

The 21 other references to this node: https://github.com/search?q=org%3Aconfluentinc+docker-oraclejdk8+NOT+is%3Aarchived&type=code

Test Strategy

PR job makes it past the existing issue

Testing done:

• [ ] Unit tests
• [ ] Integration tests
• [ ] System tests
• [ ] Manual tests

Release Plan

Merge to master and hope it works.