Bump Go versions used for testing and releasing

Releasing v0.8.0 failed[1] because of:

> failed to build for windows_arm64: exit status 2: cmd/go: unsupported
> GOOS/GOARCH pair windows/arm64

Apparently this was unsupported in Go <1.17[2], but I'm not sure why it
succeeded with the v0.7.0 release, which used 1.16...

[1]: https://github.com/grafana/xk6/actions/runs/3481621020/jobs/5822947099

[2]: golang/go#36439

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu-latest, macos-latest, windows-latest ]
-        go-version: [ 1.16.x, 1.17.x ]
+        go-version: [ 1.18.x, 1.19.x ]
 
         # Set some variables per OS, usable via ${{ matrix.VAR }}
         # XK6_BIN_PATH: the path to the compiled k6 binary, for artifact publishing

.github/workflows/release.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest ]
-        go-version: [ 1.16.x ]
+        go-version: [ 1.19.x ]
     runs-on: ${{ matrix.os }}
 
     steps: