fix: remove OpenSSL dependency by migrating isahc to reqwest (rustls-tls)

This commit fixes CI failures caused by curl-sys requiring OpenSSL 3.0.0+
which is not available in the CI container image (Debian bullseye).

Changes:
- Replace isahc with reqwest (rustls-tls feature) in relay-utils
- Remove isahc from workspace dependencies
- Update reqwest to use rustls-tls and json features
- Update Cargo.lock (removes curl, curl-sys, isahc, openssl-sys, native-tls)

Benefits:
- Pure Rust TLS implementation (no OpenSSL dependency)
- More portable across different Linux distributions
- Eliminates C compilation requirements for TLS
- Better security (memory-safe TLS implementation)

Affected workflows:
- Checks / cargo-clippy
- Checks / check-try-runtime
- Docs / test-doc, build-rustdoc
- Build and push images
- tests linux stable
- tests misc

pezbridges/relays/utils/Cargo.toml

@@ -19,7 +19,8 @@ async-std = { workspace = true }
 async-trait = { workspace = true }
 backoff = { workspace = true }
 futures = { workspace = true }
-isahc = { workspace = true }
+# Using reqwest with rustls-tls for pure Rust TLS (no OpenSSL dependency)
+reqwest = { workspace = true }
 jsonpath_lib = { workspace = true }
 num-traits = { workspace = true, default-features = true }
 parking_lot = { workspace = true, default-features = true }