release(15.0.0): updated release notes & version

[erisu]

Platforms affected

n/a

Motivation and Context

Prepare 15.0.0 release

Description

• Update Version
• Update RELEASENOTES

Testing

n/a

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.39%. Comparing base (b4eadcc) to head (ce23402).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1885   +/-   ##
=======================================
  Coverage   61.39%   61.39%           
=======================================
  Files          24       24           
  Lines        4919     4919           
=======================================
  Hits         3020     3020           
  Misses       1899     1899           

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