Update collection-index.yml

[igwejk]

List demos-by-igwejk/devcontainer-templates/java template

What type of PR is this?

• [x] Add a new dev container collection
• [ ] Update to an existing dev container collection
• [ ] Documentation/spec update
• [ ] Other containers.dev site update (UX, layout, etc)

Related Issues

• Related Issue #
• Closes #

Description

Please replace this line with a description of your PR.

Collection checklist

If your PR contributes a new collection, please utilize this checklist:

• [x] Collection name
• [x] Maintainer name
• [x] Maintainer contact link (i.e. link to a GitHub repo, email)
• [x] Repository URL
• [x] OCI Reference
• [x] I acknowledge that this collection provides new functionality, distinct from the existing collections part of this index.