Fix camera_info_topic default value

[iche033]

🦟 Bug fix

Summary

The <camera_info_topic> default value was set to __default__ in sdf 1.7 in #1201. However, these changes were not forward ported to sdf 1.8, 1.9. Ran into this bug as it caused a test failure in gz-sensors6 when I tried to forward port gz-sensors3 to 6 in gazebosim/gz-sensors#327.

I believe __default__ should be the correct value since the code specifically looks for this string: https://github.com/gazebosim/sdformat/blob/sdf12/src/Camera.cc#L263

Checklist

• Signed all commits for DCO
• Added tests
• 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 #1241 (13a6ac9) into sdf12 (4121eaf) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 13a6ac9 differs from pull request most recent head 5654e2a. Consider uploading reports for the commit 5654e2a to get more accurate results

@@           Coverage Diff           @@
##            sdf12    #1241   +/-   ##
=======================================
  Coverage   92.02%   92.03%           
=======================================
  Files          79       79           
  Lines       13090    13090           
=======================================
+ Hits        12046    12047    +1     
+ Misses       1044     1043    -1     

Impacted Files	Coverage Δ
src/Camera.cc	89.64% <0.00%> (+0.17%)	⬆️

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