Migrate to Qt6

[azeey]

🎉 New feature

Closes gazebosim/gz-gui#586

Requires gazebosim/gz-gui#657

Summary

Based on #2305

• See Migrate to Qt6 gz-gui#657 for more details
• b2ffebc was needed to get scene updates since registering SerializedStepMap as a qt type wasn't working properly. The new approach seems simpler so I think we should use it as our permanent solution.

Test it

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)
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