Fix soft shadow sampling offsets on mobile.

[eh-unity]

Purpose of this PR

A simple and easily backportable fix for soft shadows on mobile (case 1374960).
Fixes soft shadows on mobile platforms (they were "hard" shadows by accident.)
Mobile soft shadow quality is still reduced compared to desktop for performance reasons (expected).

Previously:

After fix:

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Testing status

Manually and visually tested on a sample scene.
Ran URP Foundation tests locally.

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Comments to reviewers

Backported to 21.2

[github-actions]

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