Find gz-gui without major version

[scpeters]

🎉 New feature

Part of gazebo-tooling/release-tools#1244, companion to gazebosim/gz-gui#670.

Summary

The major version is removed from the cmake project name for gz-gui in gazebosim/gz-gui#670, so update the find_package and target_link_libraries calls accordingly.

Test it

• Build Remove major version from package name gz-gui#670, Find gz-gui without major version gz-sim#2843, and Find gz-gui without major version gz-launch#293 from source
• Compile this branch against them
• Confirm this builds successfully

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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