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.

Cargo.lock

@@ -2223,6 +2223,7 @@ dependencies = [
  "aws-sdk-s3",
  "axum",
  "axum-extra",
+ "base64 0.13.1",
  "call",
  "channel",
  "chrono",
@@ -2272,6 +2273,7 @@ dependencies = [
  "settings",
  "sha2 0.10.7",
  "sqlx",
+ "subtle",
  "telemetry_events",
  "text",
  "theme",