Remove deprecated image data api

[luca-della-vedova]

🦟 Bug fix

Summary

Same as the gz-gui PR. Migrate to new and safer Image API to avoid deprecation warnings.
Depends on gz-common PR.

In the process I actually found out the test that was using the deprecated API was actually doing nothing since it was comparing an image to itself. I fixed that so the test should now do what it's supposed to, with the consequence that it actually fails since I guess bytewise equality in a whole image is quite a strict constraint.
I'm not too familiar with that system so I'm not sure if the test can be fixed or should be disabled.

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)
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[luca-della-vedova]

Replaced by #1570.
Will be effective as of next gazebo6->main forward port