[HDRP] Update ray tracing documentation limitation showing unity version number

[remi-chapelain]

Purpose of this PR

Ray Tracing documentation was showing the limitation in 2020.2 but the limitations are still applied in recent versions, so no need to show version number here.

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics
Search for your PR branch using the sidebar on the left, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

HDRP
/.yamato%252Fall-hdrp.yml%2523PR_HDRP_trunk
With changes to HDRP packages, you should also run
/.yamato%252Fall-lightmapper.yml%2523PR_LightMapper_trunk

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