Add support for High Contrast Mode

**Is your feature request related to a problem? Please describe.**
ERT has no support for high contrast mode, making the user experience worse for people who rely on this.

**Describe the solution you'd like**
PyQT6.10 and above have implemented full support for high contrast, we should upgrade to this and utilize the funcitonality. This is however blocked by not beeing available on RHEL8

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Relates to #12974 somewhat, where we removed a hacky way of detecting high contrast mode.

Check the project's PyQt dependency configuration and RHEL8 compatibility first; the issue does not name a file or test. Compare the current high-contrast handling with #12974 and verify that PyQt6.10+ functionality can be used without breaking RHEL8 support. Done means ERT supports high-contrast mode.

Written by the indexing model from the issue text.