Bump Go module compatibility to Go 1.17 (#284)

This will match the README documentation, especially now that Go 1.18 is released.

Updated via:

```
go mod edit -go=1.17
go mod tidy
```

go.mod

@@ -1,6 +1,6 @@
 module github.com/hashicorp/terraform-exec
 
-go 1.14
+go 1.17
 
 require (
 	github.com/andybalholm/crlf v0.0.0-20171020200849-670099aa064f
@@ -13,3 +13,28 @@ require (
 	github.com/zclconf/go-cty v1.10.0
 	github.com/zclconf/go-cty-debug v0.0.0-20191215020915-b22d67c1ba0b
 )
+
+require (
+	github.com/Microsoft/go-winio v0.4.16 // indirect
+	github.com/ProtonMail/go-crypto v0.0.0-20210428141323-04723f9f07d7 // indirect
+	github.com/acomagu/bufpipe v1.0.3 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/emirpasic/gods v1.12.0 // indirect
+	github.com/go-git/gcfg v1.5.0 // indirect
+	github.com/go-git/go-billy/v5 v5.3.1 // indirect
+	github.com/go-git/go-git/v5 v5.4.2 // indirect
+	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect
+	github.com/imdario/mergo v0.3.12 // indirect
+	github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99 // indirect
+	github.com/kevinburke/ssh_config v0.0.0-20201106050909-4977a11b4351 // indirect
+	github.com/mitchellh/go-homedir v1.1.0 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/xanzy/ssh-agent v0.3.0 // indirect
+	golang.org/x/crypto v0.0.0-20210616213533-5ff15b29337e // indirect
+	golang.org/x/net v0.0.0-20210326060303-6b1517762897 // indirect
+	golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1 // indirect
+	golang.org/x/text v0.3.5 // indirect
+	golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 // indirect
+	gopkg.in/warnings.v0 v0.1.2 // indirect
+	gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c // indirect
+)

go.sum

@@ -19,7 +19,6 @@ github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/emirpasic/gods v1.12.0 h1:QAUIPSaCu4G+POclxeqb3F+WPpdKqFGlw36+yOzGlrg=
 github.com/emirpasic/gods v1.12.0/go.mod h1:YfzfFFoVP/catgzJb4IKIqXjX78Ha8FMSDh3ymbK86o=
-github.com/flynn/go-shlex v0.0.0-20150515145356-3f9db97f8568 h1:BHsljHzVlRcyQhjrss6TZTdY2VfCqZPbv5k3iBFa2ZQ=
 github.com/flynn/go-shlex v0.0.0-20150515145356-3f9db97f8568/go.mod h1:xEzjJPgXI435gkrCt3MPfRiAkVrwSbHsst4LCFVfpJc=
 github.com/gliderlabs/ssh v0.2.2 h1:6zsha5zo/TWhRhwqCD3+EarCAgZ2yN28ipRnGPnwkI0=
 github.com/gliderlabs/ssh v0.2.2/go.mod h1:U7qILu1NlMHj9FlMhZLlkCdDnU1DBEAqr0aevW3Awn0=