Backport cmake/test fixes to sdf6

[scpeters]

🦟 Bug fix

Fixes issues on sdf6 branch when built on Ubuntu 20.04, as exposed by testing for gazebo-tooling/release-tools#565 and backports terminology fixes in Changelog

Summary

There is a cmake warning and test failure when building sdf6 on Ubuntu 20.04:

• https://build.osrfoundation.org/job/sdformat-ci-sdformat6-focal-amd64/1/

These have already been fixed in later versions of libsdformat:

• https://github.com/ignitionrobotics/sdformat/pull/328 test failure was fixed in Find python3 in cmake, fix warning #328
• Value of sdf_headers contained a newline cmake warning was fixed by 0c972bd that was squashed in Don't install deprecated parser_urdf.hh header #276

Also, I back ported the use of libsdformat and SDFormat in the Changelog

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)
• 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

[chapulina]

The changes look good to me. The homebrew warning is already present on sdf6:

https://build.osrfoundation.org/job/sdformat-ci-sdformat6-homebrew-amd64/35/