build: Bump go.mod Go version to 1.14 (#356)

This is complimentary to dropping support for Go 1.13.
We can now declare the minimum Go version to be 1.14.

We still support the last three stable Go releases, which is more than
what the Go project and community normally do (the norm is to support
the last two stable releases).

https://golang.org/doc/modules/gomod-ref

go.mod

@@ -1,6 +1,6 @@
 module github.com/getsentry/sentry-go
 
-go 1.13
+go 1.14
 
 require (
 	github.com/ajg/form v1.5.1 // indirect