Bump Go tool chain to 1.20

This change was made to address a number of vulnerabilities reported by
govulncheck

```
~>  govulncheck ./...
Scanning your code and 599 packages across 99 dependent modules for known vulnerabilities...

Vulnerability #1: GO-2023-2043
    Improper handling of special tags within script contexts in html/template
  More info: https://pkg.go.dev/vuln/GO-2023-2043
  Standard library
    Found in: html/[email protected]
    Fixed in: html/[email protected]
    Example traces found:
      #1: multistep/commonsteps/step_http_server.go:123:2: commonsteps.StepHTTPServer.Run calls http.Server.Serve, which eventually calls template.Template.Execute
      #2: multistep/commonsteps/step_http_server.go:123:2: commonsteps.StepHTTPServer.Run calls http.Server.Serve, which eventually calls template.Template.ExecuteTemplate

Vulnerability #2: GO-2023-2041
    Improper handling of HTML-like comments in script contexts in html/template
  More info: https://pkg.go.dev/vuln/GO-2023-2041
  Standard library
    Found in: html/[email protected]
    Fixed in: html/[email protected]
    Example traces found:
      #1: multistep/commonsteps/step_http_server.go:123:2: commonsteps.StepHTTPServer.Run calls http.Server.Serve, which eventually calls template.Template.Execute
      #2: multistep/commonsteps/step_http_server.go:123:2: commonsteps.StepHTTPServer.Run calls http.Server.Serve, which eventually calls template.Template.ExecuteTemplate

Your code is affected by 2 vulnerabilities from the Go standard library.

```

go.mod

@@ -117,6 +117,6 @@ require (
 	google.golang.org/protobuf v1.28.1 // indirect
 )
 
-go 1.19
+go 1.20
 
 retract v0.5.0 // v0.5.0 of the SDK was broken because of the replace statement for go-cty