Fix Flashbots relay endpoint (#888)

## 📝 Summary

Previously included endpoint was internal without stability guarantees,
switch to public-facing one.

## 💡 Motivation and Context

<!--- (Optional) Why is this change required? What problem does it
solve? Remove this section if not applicable. -->

---

## ✅ I have completed the following steps:

* [ ] Run `make lint`
* [ ] Run `make test`
* [ ] Added tests (if applicable)

Author: ilyaluk

crates/rbuilder/src/live_builder/config.rs

@@ -947,7 +947,7 @@ lazy_static! {
             "flashbots".to_string(),
             RelayConfig {
                 name: "flashbots".to_string(),
-                url: "http://k8s-default-boostrel-9f278153f5-947835446.us-east-2.elb.amazonaws.com"
+                url: "https://0xac6e77dfe25ecd6110b8e780608cce0dab71fdd5ebea22a16c0205200f2f8e2e3ad3b71d3499c54ad14d6c21b41a37ae@boost-relay.flashbots.net"
                     .to_string(),
                 grpc_url: None,
                 mode: RelayMode::Full,