Update release notes instructions

[carlossanlop]

The old instructions had an obsolete script that was not collecting all the changes between releases. Sometimes it was not collecting any.

I generated these release notes using these instructions:

• https://github.com/dotnet/roslyn-analyzers/releases/tag/7.0.3
• https://github.com/dotnet/roslyn-analyzers/releases/tag/7.0.4
• https://github.com/dotnet/roslyn-analyzers/releases/tag/8.0.0
• https://github.com/dotnet/roslyn-analyzers/releases/tag/9.0.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.50%. Comparing base (5ed3367) to head (a3777ff).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7503   +/-   ##
=======================================
  Coverage   96.50%   96.50%           
=======================================
  Files        1452     1452           
  Lines      347566   347566           
  Branches    11416    11416           
=======================================
+ Hits       335413   335423   +10     
+ Misses       9257     9248    -9     
+ Partials     2896     2895    -1     

[carlossanlop]

@arunchndr can you PTAL?