Disable generation of the public CI pipelines in Java

When the public CI pipelines are disabled for Java and the pullrequest pipeline is the only public pipeline, the
[prepare-pipelines.yml](https://github.com/Azure/azure-sdk-tools/blob/main/eng/common/pipelines/templates/jobs/prepare-pipelines.yml#L90) for Java needs to have the following field added
`$generatePublicCIPipeline = 'false'`

Note: Below Java you can see that JS, Python and Rust have already done this.