Remove GTK2 Linux prerequisites

[MikeMcC399]

Situation

Linux Prerequisites lists both GTK 2 and GTK 3 packages for installation.

• PR fix: force gtk version 3 to avoid Electron 36 from crashing cypress#32372 locks Electron usage to GTK 3
• https://www.gtk.org/ supports only GTK 3 and GTK 4. GTK 2 reached end-of-life in 2020 according to https://blog.gtk.org/2020/12/16/gtk-4-0/
• Debian-based Cypress Docker images built using factory/factory.Dockerfile include only libgtk-3-0, not libgtk2.0-0

Change

Remove GTK 2 prerequisites from Linux Prerequisites

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[MikeMcC399]

@jennifer-shehane

This PR proposes to remove the unsupported GTK2 Linux prerequisites from the documentation. Cypress doesn't need them anymore. The PR is waiting for review.