Update Postman Learning link to canonical URL (currently 301 redirect)

## Redirecting link (301)
- URL: https://learning.postman.com/open-technologies/
- HTTP status: 301
- Found on: https://microcks.io/blog/microcks-joining-cncf-sandbox/

## What to do
- Follow the redirect and replace with the final canonical URL.

## Help needed
If you can’t determine the final destination (or best replacement), comment here and we’ll help.
