[2021.2] Bump SRP packages to 12.1.0

[theopnv]

Purpose of this PR

Backport of #5812.

• Bumped SRP packages to 13.1.0
• Moved some release notes to the new section, as they were added after the commit used for the current vendoring (and therefore correspond to the next vendoring).

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Testing status

Pack & Test all packages (upm-ci validation): https://unity-ci.cds.internal.unity3d.com/project/902/branch/2021.2%2Fsrp%2Fbump-packages/jobDefinition/.yamato%2F_projectcontext.yml%23all_package_ci_project_2021.2

[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

URP
/.yamato%252Fall-urp.yml%2523PR_URP_2021.2

Shader Graph
/.yamato%252Fall-shadergraph.yml%2523PR_ShaderGraph_2021.2
Depending on your PR, you may also want
/.yamato%252Fall-shadergraph_builtin_foundation.yml%2523PR_ShaderGraph_BuiltIn_Foundation_2021.2
/.yamato%252Fall-shadergraph_builtin_lighting.yml%2523PR_ShaderGraph_BuiltIn_Lighting_2021.2

VFX
/.yamato%252Fall-vfx.yml%2523PR_VFX_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]

there is also documentation.cs to bump and version.hlsl. (yes bumping is a nightmare :) )

[theopnv]

Applied requested changes @sebastienlagarde, thanks.
Removed all modifications made to the templates (except the hdrp or urp version they're using), as you are already bumping them in other PRs #5799.