chore(pf5): use new background image for About modal

[tthvo]

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Related to #1303

Description of the change:

Added a new SVG asset to use as the About modal background. Since last time, I made the SVG dimmer (less opacity) to closely match the style of built-in SVG and cropped it a bit more.

Motivations

See #1303 (comment)

Others

I used this simple SVG editor from FlatHub: https://boxy-svg.com/

[andrewazores]

Looks good, but tests are failing.

[tthvo]

Looks good, but tests are failing.

Unfortunately, tests are failing everywhere on our pf5 base branch :(( lets fix it all together in a PR?

[andrewazores]

Ah right, I didn't notice that this was targeting pf5.