chore: add link to modernize a legacy cobol app exercise (skills#84)

Author: FidelusAleksander

.github/steps/x-review.md

@@ -20,6 +20,7 @@ Here's a recap of your accomplishments:
   - Use Copilot with different models.
 - Check out the other [GitHub Skills exercises](https://learn.github.com/skills).
   - Learn how to [Integrate MCP with Copilot](https://github.com/skills/integrate-mcp-with-copilot)
+  - Tackle legacy COBOL code in [Modernize Your Legacy Code with GitHub Copilot](https://github.com/skills/modernize-your-legacy-code-with-github-copilot) exercise
   - Try GitHub Copilot Coding Agent in the [Expand your team with Copilot](https://github.com/skills/expand-your-team-with-copilot) exercise
 
 Check out these resources to learn more about GitHub Copilot :