fix: remove the info about gRPC instrumentation readiness

Author: michal-kazmierczak

instrumentation/grpc/README.md

@@ -1,6 +1,4 @@
 # OpenTelemetry gRPC Instrumentation
-> [!CAUTION]
-> This gem is not ready for release by the open-telemetry org yet, but other versions are available on RubyGems
 
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