Improve version table

[martincostello]

• Use named links to make it easier to add things and to make the table easier to read when making edits.
• Fix some incorrect links.

[Copilot]

Pull Request Overview

This PR refactors the compatibility version table in the README to use named Markdown link references instead of inline URLs, making it easier to maintain and more readable when editing.

• Converted inline badge and link URLs to named references
• Fixed incorrect version tags in Microsoft.OpenApi release links (missing 'v' prefix for v8 and v7)
• Updated vnext and latest OpenAPI version sources to use .csproj files instead of Directory.Packages.props

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.53%. Comparing base (11db6ef) to head (a05c1e5).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3653   +/-   ##
=======================================
  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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