Update algorithm for specular occlusion

[adrien-de-tocqueville]

Purpose of this PR

This PR is updating the algorithm for specular occlusion when using bent normals + AO.
This new version reduces specular occlusion sharpness for high smoothness materials, and reduces over darkenning of surfaces when looking at grazing angles.

Also changed default value for specular occlusion for new shader graphs. Previous was "Off", now it's "From AO" to match default value of the Lit shader

Comparison of the different options:
Before this PR is "From Bent Normals", new algorithm is "From Spherical Gaussians"
Both models have high smoothness in the gif below, all lighting is from HDRI Sky

Added a section in Shader Code of upgrade guide about function renaming.
Added screenshot in Whats new guide

Testing status

https://confluence.unity3d.com/pages/viewpage.action?spaceKey=HDRP&title=Specular+Occlusion

[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_2021.2

SRP Core
You could run ABV on your branch before merging your PR, but it will start A LOT of jobs. Please be responsible about it and run it only when you feel the PR is ready:
/.yamato%252F_abv.yml%2523all_project_ci_2021.2
Be aware that any modifications to the Core package impacts everyone in the Graphics repo so please discuss the PR with your lead.

Depending on the scope of your PR, you may need to run more jobs than what has been suggested. Please speak to your lead or a Graphics SDET (#devs-graphics-automation) if you are unsure.

[sebastienlagarde]

Note for reviever: Before merging we want to know what method make sense and then we will remove the other one. Also be sure it works with all our shader type.

[adrien-de-tocqueville]

adding QA for awareness but already tested by artists 🙂

[sebastienlagarde]

PR tested by the artsits. No need for futher QA. Merging