Precompression: replace gzip by zstd

[LeoniePhiline]

Nowadays, all relevant browsers support brotli:

https://caniuse.com/?search=brotli

Supporting precompressed gzip should not be required any more.

Instead, cargo-leptos might want to instead precompress to the currently leading and widely supported compression algorithm, zstd:

https://caniuse.com/?search=zstd

Since not all relevant browsers support Content-Encoding: zstd yet, brotli is still required.

Proposal:

• cargo leptos build --release --precompress precompresses using brotli and zstd, dropping gzip.
• leptos_axum::file_and_error_handler_with_context configures the tower ServeDir to serve files precompressed with brotli and zstd.