chore: bump itest go toolchain to 1.26.3

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: korniltsev-grafanista

itest/go.mod

@@ -2,6 +2,8 @@ module otel-profiling-java-itest
 
 go 1.25.0
 
+toolchain go1.26.3
+
 require (
 	connectrpc.com/connect v1.19.1
 	github.com/dennwc/varint v1.0.0