[URP][Backport] make the Vulkan RenderTexture MSAA support fallback work in the same way as the swapchain backend implementation

[manuele-bonanno]

backport of:
#6180
#2342

Backport case:
https://fogbugz.unity3d.com/f/cases/1384313/

Purpose of this PR

The way RenderTextures handle MSAA fallback when an unsupported sample count of 2 is requested (falling back to numSamples = 1), differs fom the way the fallback is handled when setting up the Vulkan swapchain (rounding up numSamples to 4, if supported). This caused an issue on Mali GPUs which don't support 2x MSAA.
This PR makes sure that on Vulkan the MSAA unsupported fallback behaviour is consistent between RenderTextures and Swapchain.

We are also making sure that depth copy is always disabled on GLES, since it doesn't work on that platform.

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Testing status

Tested the repro project of this case.

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Comments to reviewers

we should review how all backends handle MSAA fallbacks and move these implementation details in engine code.

check the implementation of SelectSupportedSampleCount(const VkSampleCountFlags supportedSamplesCounts, const VkSampleCountFlagBits requested) in GfxDeviceVK.cpp for more info

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