Bump the go version to 1.18

1.17 has runtime errors:

```
fatal error: checkptr: converted pointer straddles multiple allocations
```

GO11MODULE behavior changed in 1.17 so README.md is updated accordingly.

.github/workflows/ci.yaml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-18.04]
-        go: [1.17, 1.18]
+        go: [1.19, 1.18]
     steps:
     - uses: actions/checkout@v2
     - uses: actions/setup-go@v2

README.md

@@ -41,15 +41,11 @@ by running:
     apt-get install golang
     apt-get install libnl-3-dev libnl-genl-3-dev
 
-If your distro has a go version before 1.17, you may need to fetch a newer
+If your distro has a go version before 1.18, you may need to fetch a newer
 release from https://golang.org/dl/.
 
-If you are running go version 1.11 and above, you can use go modules to avoid
-installing go packages manually. By go 1.12, `GO111MODULE` defaults to `auto`,
-so remember to enable go module by `export GO111MODULE=on`.
-
-If you are running before go version 1.11 or you don't want to enable
-GO111MODULE, after setting `GOPATH` to an appropriate location (for example `~/go`):
+If you are running before go version 1.11 or you want to set `GO111MODULE=off`,
+after setting `GOPATH` to an appropriate location (for example `~/go`):
 
     go get -u golang.org/x/crypto/ssh
     go get -u github.com/dlintw/goconf

go.mod

@@ -1,6 +1,6 @@
 module github.com/google/seesaw
 
-go 1.17
+go 1.18
 
 require (
 	github.com/dlintw/goconf v0.0.0-20120228082610-dcc070983490