[ET-VK][ez] Fix 8-bit linear shaders extension requirement (#7607)

## Context

8-bit linear compute shaders only require the 8bit_storage extension for buffer tensors. Now that we are checking that the GPU supports required extensions, we need to be more careful about declaring that a extensions as required.

Differential Revision: [D68035429](https://our.internmc.facebook.com/intern/diff/D68035429/)

ghstack-source-id: 260953913
Pull Request resolved: https://github.com/pytorch/executorch/pull/7600

Co-authored-by: Stephen Jia <ssjia@meta.com>