chore!: ensure consistent secret token generation and hashing (#20388)

This PR uses the same sha256 hashing technique as we use for APIKeys. So
now all randomly generated secrets will be hashed with sha256 for
consistency.

This is a breaking change for the oauth tokens. Since oauth is only
allowed for dev builds and experimental, this is ok.

coderd/apidoc/docs.go

@@ -15471,7 +15471,10 @@ const docTemplate = `{
                     }
                 },
                 "registration_access_token": {
-                    "type": "string"
+                    "type": "array",
+                    "items": {
+                        "type": "integer"
+                    }
                 },
                 "registration_client_uri": {
                     "type": "string"