build: Reset go.mod version to 1.11

Currently the go tool use the Go version defined in go.mod to check for
uses of features that did not exist prior to the specified version.

Additionally, when a build fails, that version may end up in the error
message.

To prevent confusion for SDK users, set the version to the oldest Go
release supported by sentry-go.

go.mod

@@ -1,6 +1,6 @@
 module github.com/getsentry/sentry-go
 
-go 1.13
+go 1.11
 
 require (
 	github.com/ajg/form v1.5.1 // indirect