Bump Go version to fix macOS resolver issue

With Go version < 1.20, Go is using the internal Go DNS resolver by default
which only considers the file /etc/resolv.conf similar to glibc's behaviour.
But macOS also supports additional configurations under /etc/resolver which
is often used for local development to use proper name resolution.

The behaviour has been changed beginning with Go version 1.20
(see also https://go-review.googlesource.com/c/go/+/446178)

Fixes hashicorp#1940

.go-version

@@ -1 +1 @@
-1.19.4
+1.20.6

README.md

@@ -23,7 +23,7 @@ Requirements
 ------------
 
 - [Terraform](https://www.terraform.io/downloads.html) 0.12.x and above, we recommend using the latest stable release whenever possible.
-- [Go](https://golang.org/doc/install) 1.19 (to build the provider plugin)
+- [Go](https://golang.org/doc/install) 1.20 (to build the provider plugin)
 
 Building The Provider
 ---------------------
@@ -45,7 +45,7 @@ $ make build
 Developing the Provider
 ---------------------------
 
-If you wish to work on the provider, you'll first need [Go](http://www.golang.org) installed on your machine (version 1.19+ is *required*). You'll also need to correctly setup a [GOPATH](http://golang.org/doc/code.html#GOPATH), as well as adding `$GOPATH/bin` to your `$PATH`.
+If you wish to work on the provider, you'll first need [Go](http://www.golang.org) installed on your machine (version 1.20+ is *required*). You'll also need to correctly setup a [GOPATH](http://golang.org/doc/code.html#GOPATH), as well as adding `$GOPATH/bin` to your `$PATH`.
 
 To compile the provider, run `make build`. This will build the provider and put the provider binary in the `$GOPATH/bin` directory.