Clarify compatibility

[martincostello]

• Update ASP.NET Core versions.
• Clarify that Microsoft.AspNetCore.OpenApi is the only part of ASP.NET Core dependent on Microsoft.OpenApi.
• Remove v7 from the compatibility table as ASP.NET Core 6.0 is now out-of-support.
• Add link to SemVer website.

Resolves #3693.

[Copilot]

Pull request overview

This PR updates the compatibility documentation to reflect current ASP.NET Core version support and clarifies dependency relationships. The changes remove outdated information about ASP.NET Core 6.0 (now out-of-support) and improve the accuracy of technical explanations.

• Added a link to the Semantic Versioning website for better context
• Clarified that only the Microsoft.AspNetCore.OpenApi NuGet package has a dependency on Microsoft.OpenApi v2+
• Removed v7 compatibility information from the table as ASP.NET Core 6.0 is no longer supported

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/migrating-to-v10.md	Added SemVer link reference and clarified that Microsoft.AspNetCore.OpenApi package specifically depends on Microsoft.OpenApi v2+
README.md	Removed v7 compatibility row and updated ASP.NET Core version specifications from >= 8.0.0 to explicit version ranges (9.0.x, 8.0.x) for v9 and v8

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.66%. Comparing base (4ae817b) to head (1ce259b).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3694   +/-   ##
=======================================
  Coverage   94.66%   94.66%           
=======================================
  Files         111      111           
  Lines        3858     3858           
  Branches      778      778           
=======================================
  Hits         3652     3652           
  Misses        206      206           

Flag	Coverage Δ
Linux	94.66% <ø> (ø)
Windows	94.66% <ø> (ø)
macOS	94.66% <ø> (ø)

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