Update migration guide

[martincostello]

Link to the specific documentation.

Relates to #3648 and #3651.

[Copilot]

Pull Request Overview

This PR updates the v10 migration guide to include a link to specific documentation about the AddSecurityRequirement() API change. The change helps developers understand how to migrate to the new Func<OpenApiDocument, OpenApiSecurityRequirement> signature.

• Adds inline documentation reference to AddSecurityRequirement() migration note
• Adds link reference pointing to the security configuration documentation

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.53%. Comparing base (b96186f) to head (9a42da9).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3654   +/-   ##
=======================================
  Coverage   94.53%   94.53%           
=======================================
  Files         111      111           
  Lines        3861     3861           
  Branches      778      778           
=======================================
  Hits         3650     3650           
  Misses        211      211           

Flag	Coverage Δ
Linux	94.53% <ø> (ø)
Windows	94.53% <ø> (ø)
macOS	94.53% <ø> (ø)

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