Update Flatpak platform to Gnome 42

[darkdragon-001]

Before Posting:

• [x] Create an issue describing the change. If an issue already exists, please use this.
• [x] Discuss the issue with the HandBrake team to ensure that the idea has been accepted. (An open enhancement request does not equal acceptance). This will avoid any time wasted on features that are outside the project scope!

See flathub/fr.handbrake.ghb#19

Description of Change:

Update Gnome platform in Flatpak manifest to version 42.

Test on:

• [ ] Windows 10+ (via MinGW)
• [ ] macOS 10.13+
• [x] Ubuntu Linux

Screenshots (If relevant):

Log file output (If relevant):

[darkdragon-001]

Gnome 43 will be released on September 21st. So if your release comes later, make sure to update to that version directly :wink: