fix: Authn pages - fix background on medium resolutions [quince]

[Lunyachek]

This is backport from master branch - #1192

Description

At some resolutions, we can see this bug. It's related to the SVG image having a specific height, and at certain page sizes, this height isn't enough for the SVG to reach the bottom of the page. We suggest drawing a slanted background using CSS

How Has This Been Tested?

Locally, in different browsers and resolutions (Screenshots below)

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After

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