all: note v5.9.0 release and update otlp/go.mod

Author: kevinburkesegment

HISTORY.md

@@ -1,6 +1,6 @@
 # History
 
-### v5.9.0 (Unreleased)
+### v5.9.0 (June 15, 2026)
 
 **The minimum supported Go version is now 1.25.** The OpenTelemetry SDK and gRPC
 both declare `go 1.25.0`, and the `otlp` package lives in the main module, so