Find gz-launch without major version

[scpeters]

🎉 New feature

Part of gazebo-tooling/release-tools#1244, companion to gazebosim/gz-launch#286

Summary

The major version is removed from the cmake project name for gz-launch in gazebosim/gz-launch#286, so update the find_package call accordingly.

Test it

• Build Remove major version from package name gz-launch#286 from source
• Compile this branch against it
• 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

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.