[Qt6] Harsh shadow around VPN boxes / modals

[data-sync-user]

Important details: 

• also reproducing for the rest of the modals / nav bar/ boxes (from Privacy features screen, etc); 
• this seems to be a Qt issue as it is not reproducing with Qt 6.2.4 builds on the rest of the platforms;

Version / build found in;

• 2.21.0 (2.21.0~build20240212-noble1) - Qt version 6.4.2;

• 2.21.0 (2.21.0~build20240212-mantic1) - Qt version 6.4.2;

Reproducible on:

• Ubuntu 23.10 / Ubuntu 24.04; 

Prerequisites:

• Have Mozilla VPN app installed and open;

• Have a Mozilla account with an active subscription;

• Be signed in;

Steps to reproduce:

1. Go to Settings - My devices and open the “Remove device?” modal while observing the modal shadow;
2. Go to Settings - Get help - Reset VPN and open the “Confirm reset” modal while observing the modal shadow;
3. Go to Settings - About us - Check for updates and observe the modal shadow;
4. Observe the onboarding screen;

Expected result:

• There is a soft shadow around modals;

Actual result:

• There is a harsh shadow around modals;

Notes:

• See the attached ss;

┆Issue is synchronized with this Jira Bug ┆Reporter: Bianca Hidecuti