[Master] Adding [Gamma] tag to the MainColor property inside ParticlesUnlit and Unlit shaders

[ellioman]

Purpose of this PR

In #2146 I introduced a regression, causing the main color to change in materials using Unlit and Particles Unlit shaders.
This affected several of our CI tests.

The fix is to add the [Gamma] tag to those properties.

Yamato

https://unity-ci.cds.internal.unity3d.com/job/8350051

[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)

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

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.

[github-actions]

It appears that you made a non-draft PR!
Please convert your PR to draft (button on the right side of the page).
See the PR template for more information.
Thank you!

[Verasl]

LGTM

[ellioman]

Closed as this PR and will revert the original changes as well.
This requires a bit more work to find a better solution which we've noted down.