deprecate inertia

[nkoenig]

🎉 New feature

Summary

Deprecated the Inertia class, which is not used in sdformat.

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

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #1019 (c3b7b9c) into main (f3457ef) will not change coverage.
The diff coverage is n/a.

❗ Current head c3b7b9c differs from pull request most recent head 7d6bc72. Consider uploading reports for the commit 7d6bc72 to get more accurate results

@@           Coverage Diff           @@
##             main    #1019   +/-   ##
=======================================
  Coverage   83.64%   83.64%           
=======================================
  Files         146      146           
  Lines       18255    18255           
=======================================
  Hits        15269    15269           
  Misses       2986     2986           

Impacted Files	Coverage Δ
include/sdf/Types.hh	100.00% <ø> (ø)

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