Upgrade conda to require qt 5.15

[martyngigg]

Describe the outcome that is desired. By the end of this work the codebase should be able to run with Qt 5.15. This should hopefully solve #34081 without us needing to change our code.

Describe any solutions you are considering Update the developer and package conda-recipes to require Qt 5.15 as a minimum.

Additional context 5.15 is the final Qt 5 release. It is in support mode and still receiving bugfixes. We cannot yet move to Qt 6 until conda-forge make it available.