Changed Ambient Mode to Dynamic by default

[JulienIgnace-Unity]

Purpose of this PR

Change the defaults of the Visual Environment component's Ambient mode to "Dynamic".
The idea is that by default it will provide users a more "out of the box" experience where ambient lighting works without any manual action from them.

This should not break any existing project apart from one very specific case: If the user's project does not have a default volume profile already containg a visual environment component, a new one will be created by HDRP automatically in memory and will then use Dynamic Ambient contrary to default static.
I feel that this is not too much of a problem because very few projects will have that issue since we always create one by default.

Issue raised in this case https://fogbugz.unity3d.com/f/cases/1357625/

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Testing status

• Created a new Visual Env component and checked that Ambient was indeed Dynamic
• Tested that existing components were not changed.

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[anisunity]

Trivial enough, lgtm

[remi-chapelain]

This change has already been discussed a few time within HDRP QA and besides the few downside that comes with it, still an improvement, especially for new users. ✔️