Modify the minimum supported version of go to 1.15

laminar · laminar · commit 0091a42dd835 · 2021-05-26T17:32:59.000+08:00
Signed-off-by: laminar &lt;fangtian@yunify.com&gt;
diff --git a/go.mod b/go.mod
@@ -1,6 +1,6 @@
 module github.com/OpenFunction/functions-framework-go
 
-go 1.11
+go 1.15
 
 require (
 	github.com/cloudevents/sdk-go/v2 v2.4.1
