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

[adlarkin]

Signed-off-by: Ashton Larkin 42042756+adlarkin@users.noreply.github.com

🦟 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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[codecov]

Codecov Report

Merging #606 (3104889) into main (baf046b) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #606      +/-   ##
==========================================
- Coverage   53.60%   53.60%   -0.01%     
==========================================
  Files         214      214              
  Lines       21353    21355       +2     
==========================================
+ Hits        11447    11448       +1     
- Misses       9906     9907       +1     

Impacted Files	Coverage Δ
ogre/src/OgreMeshFactory.cc	1.03% <0.00%> (-0.01%)	⬇️
ogre2/src/Ogre2MeshFactory.cc	78.98% <100.00%> (+0.07%)	⬆️

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