PCSS samples depth with linear sampler which is poorly supported

## Bevy version and features

main

## What went wrong

If `experimental_pbr_pcss` is enabled, shadow textures are sampled with linear sampler, which is disallowed on webgpu and may devices, and this is a wgpu bug that validation is missing.

See also https://github.com/bevyengine/bevy/issues/23965

We can either emulate linear filtering sampling, or use other methods (comparison samplers support linear filtering) but I'm not sure.