feat: upgrade OpenMLS 0.5 → 0.8 for security patches and GREASE support

Migrates all MLS code in quicprochat-core from OpenMLS 0.5 to 0.8:
- StorageProvider replaces OpenMlsKeyStore (keystore.rs full rewrite)
- HybridCryptoProvider updated for new OpenMlsProvider trait
- Group operations updated for new API signatures
- MLS state persistence via MemoryStorage serialization
- tls_codec 0.3 → 0.4, openmls_traits/rust_crypto 0.2 → 0.5

examples/plugins/rate_limit_plugin/Cargo.lock

@@ -3,12 +3,12 @@
 version = 4
 
 [[package]]
-name = "quicproquo-plugin-api"
+name = "quicprochat-plugin-api"
 version = "0.1.0"
 
 [[package]]
 name = "rate_limit_plugin"
 version = "0.1.0"
 dependencies = [
- "quicproquo-plugin-api",
+ "quicprochat-plugin-api",
 ]