Update to Go 1.17

Go 1.17 now lazy-loads dependencies when `go.mod` is version `go17`. This does not work for us for new projects started with `hugo mod init`.

Before this commit, starting a project with Go 1.17 with `hugo mod init` and then start adding dependencies with transitive dependenies to `config.toml` would treat the transitive dependencies as new, and you would potentially get a too recent version of those.

Note that this does not effect existing projects, where all dependencies are already recorded in `go.mod`.

Fixes #8930

snap/snapcraft.yaml

@@ -31,7 +31,7 @@ parts:
 
   hugo:
     plugin: nil
-    build-snaps: [go/1.16/stable]
+    build-snaps: [go/1.17/stable]
     source: .
     override-build: |
       set -ex