Fix changelog for 6.0.0-beta.4

[adinauer]

📜 Description

Fix triple Fixes heading in 6.0.0-beta.4 changelog as well as links to issues on some of the entries.

💡 Motivation and Context

Consistency

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

#skip-changelog

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (6.x.x@332aec5). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             6.x.x    #2066   +/-   ##
========================================
  Coverage         ?   81.07%           
  Complexity       ?     3217           
========================================
  Files            ?      230           
  Lines            ?    11819           
  Branches         ?     1572           
========================================
  Hits             ?     9582           
  Misses           ?     1650           
  Partials         ?      587           

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