Upgrade rsa to v0.9.6 (#15055)

This PR upgrades the `rsa` crate to v0.9.6.

The version we were using was rather old, and for something
security-sensitive we should be using a recent version.

No behavioral changes have been made, just updates to account for
changes in the crate's API.

Release Notes:

- N/A

crates/rpc/Cargo.toml

@@ -27,7 +27,7 @@ gpui = { workspace = true, optional = true }
 parking_lot.workspace = true
 proto.workspace = true
 rand.workspace = true
-rsa = "0.4"
+rsa.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 strum.workspace = true