Replace deprecated common::SubMesh::MaterialIndex() with GetMaterialIndex()

[adlarkin]

Signed-off-by: Ashton Larkin [email protected]

🦟 Bug fix

Summary

This PR replaces deprecated calls to ignition::common::SubMesh::MaterialIndex() with the new GetMaterialIndex() method that was introduced in gazebosim/gz-common#319.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[ahcorde]

LGTM, maybe it makes sense to adapt addSubmeshFunc to use std::optional

[codecov]

Codecov Report

Merging #1436 (e21a185) into main (a5464eb) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main    #1436      +/-   ##
==========================================
- Coverage   62.37%   62.37%   -0.01%     
==========================================
  Files         317      317              
  Lines       24489    24493       +4     
==========================================
+ Hits        15276    15278       +2     
- Misses       9213     9215       +2     

Impacted Files	Coverage Δ
...ems/collada_world_exporter/ColladaWorldExporter.cc	93.52% <66.66%> (-1.29%)	⬇️

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