Always use `immutable` directive for cached assets

[mmarinchenko]

Reference: https://webhint.io/docs/user-guide/hints/hint-http-cache/

This may be a silly question, but what does the "v" key in query string have to do with immutable directive? 🙂

https://github.com/ligershark/WebOptimizer/blob/7aa6c5ea2a3d08a1049741e1f1a68464f144ecc1/src/WebOptimizer.Core/AssetMiddleware.cs#L89-L97