Update GitHub Models url to the new url

[sgoedecke]

Why are these changes needed?

For GitHub Models inference, Autogen was using the old URL. That works fine, but doesn't support newer features like custom models or paid inference. This PR updates the URL to the one used in the GitHub docs.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[sgoedecke]

@microsoft-github-policy-service agree company="GitHub"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.42%. Comparing base (64f2751) to head (a9b07db).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6759      +/-   ##
==========================================
- Coverage   80.44%   76.42%   -4.02%     
==========================================
  Files         237      343     +106     
  Lines       17864    20982    +3118     
  Branches        0      406     +406     
==========================================
+ Hits        14370    16035    +1665     
- Misses       3494     4674    +1180     
- Partials        0      273     +273     

Flag	Coverage Δ
unittests	76.42% <100.00%> (-4.02%)	⬇️

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