Migrate from scrypt to sha256. (#8969)

This reduces the server time to compute the hash from 40ms to 5µs,
which should remove this as a noticable chunk of CPU time in production.

(An attacker who has access to our database will now need only 10^54
years of CPU time instead of 10^58 to brute force a token).

Release Notes:

- Improved sign in latency by 40ms.

crates/rpc/Cargo.toml

@@ -19,7 +19,7 @@ test-support = ["collections/test-support", "gpui/test-support"]
 [dependencies]
 anyhow.workspace = true
 async-tungstenite = "0.16"
-base64 = "0.13"
+base64.workspace = true
 collections.workspace = true
 futures.workspace = true
 gpui = { workspace = true, optional = true }