Whats-New updates for v3.16.0rc0 .

[HGWright]

Description

Please describe your awesome pull request

Checklist

Important

The Iris core developers are here to help! If anything below is unclear, just post a comment asking for help 😊

• Included a What's New entry
• Incorporated type hints in any changed code
• Checked if tests need updating
• Checked if benchmarks need updating
• Checked if documentation needs updating
  • Docstrings (we love code examples!)
  • User Manual pages
• Checked if dependencies need updating
• Confirmed that the GitHub 'checks' on this PR are passing ✅
  (further reading)

---

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  https://cla-assistant.io/check/SciTools/iris?pullRequest=9999

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.42%. Comparing base (d015de6) to head (fcaac81).

Additional details and impacted files

@@           Coverage Diff            @@
##           v3.16.x    #7238   +/-   ##
========================================
  Coverage    90.42%   90.42%           
========================================
  Files           93       93           
  Lines        25812    25812           
  Branches      4795     4795           
========================================
  Hits         23340    23340           
  Misses        1693     1693           
  Partials       779      779           

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[trexfeathers]

This renders nicely, and includes everything fragment from changelog/. I just have a single request, which I feel is quite important.