Fix broken mesh examples

[ukmo-ccbunney]

Description

Closes: #6972

Fixes the use of iris.ugrid.MeshXY and iris.ugrid.Connectivity in the Mesh Conversion examples.

Also fixes missing AuxCoords in the WAVEWATCH III Mesh example.

Note: Banking this as a draft PR for now as we might want to use the planned "Add cube as component to cube" functionality proposed in #6829.

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
• (N/A) 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)

---

Tip

Things you can trigger on this PR:

• Add this label to trigger benchmarks: benchmark_this Request that this pull request be benchmarked to check if it introduces performance shifts
• Visit this URL - swapping 9999 for this PR's number - to re-trigger the CLA check:
  https://cla-assistant.io/check/SciTools/iris?pullRequest=9999

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.15%. Comparing base (ec07587) to head (c0c61c0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7168   +/-   ##
=======================================
  Coverage   90.15%   90.15%           
=======================================
  Files          91       91           
  Lines       24983    24983           
  Branches     4685     4685           
=======================================
  Hits        22524    22524           
  Misses       1682     1682           
  Partials      777      777           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.