Update workspace default image to latest

What does this MR do and why?

• Update workspace default image to latest, which adds Php, Gcc, Python and Java, with package managers
• Update workspace default image doc

References

Issue - Point GitLab default image to workspace default... (#525608 - closed)

Screenshots or screen recordings

Tested on prod example-docker-app with devfile settings here.

How to set up and validate locally

• Check out the branch
• And follow remote development doc to spin up workspaces locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.