Revert "snap: Change confinement from strict to classic"

We need to wait until classic confinement review process at
https://forum.snapcraft.io/t/request-for-classic-confinement-for-hugo-snap/19892
is completed before "confinement: classic" uploads to the Snap Store
are accepted.

This reverts commit 6f4ff1a.

snap/snapcraft.yaml

@@ -5,7 +5,7 @@ description: |
   Hugo is a static HTML and CSS website generator written in Go. It is
   optimized for speed, easy use and configurability. Hugo takes a directory
   with content and templates and renders them into a full HTML website.
-confinement: classic
+confinement: strict
 grade: devel # "devel" or "stable"
 
 apps: