Fix deprecated Qt constants

[acolwell]

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• I've followed the contributing guidelines to the best of my understanding
• I've read and understood the contributing guidelines
• I've formatted my code according to Natron's code style
• I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
  • I have updated the documentation accordingly

What does this pull request do?

Replacing deprecated Qt constants with the supported alternatives. This is a step on the path towards getting Qt6 building and does not introduce any functional changes.

Have you tested your changes (if applicable)? If so, how?

Yes. installer still builds and Tests action still builds and both still pass all their tests. I've verified that all the Qt related deprecation warning messages are gone.