Quiet libSDFormat console on --verbose 0

[chapulina]

• Related to Warn by default on unrecognized elements sdformat#1096

Summary

SDFormat only has 2 levels: quiet / loud. Let sim users at least suppress all SDF console output with zero verbosity.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov]

Codecov Report

Merging #1621 (2264631) into ign-gazebo3 (61dc78d) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@               Coverage Diff               @@
##           ign-gazebo3    #1621      +/-   ##
===============================================
- Coverage        78.02%   78.02%   -0.01%     
===============================================
  Files              254      254              
  Lines            14932    14935       +3     
===============================================
+ Hits             11651    11653       +2     
- Misses            3281     3282       +1     

Impacted Files	Coverage Δ
src/ign.cc	72.25% <75.00%> (-0.12%)	⬇️

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