snap: Use interface names etc-gitconfig and gitconfig

as recommended by @alexmurray to keep consistency with other snaps; see
https://forum.snapcraft.io/t/system-files-and-personal-files-request-for-hugo/30598/2

One closer step towards fixing #6226

snap/snapcraft.yaml

@@ -18,11 +18,11 @@ package-repositories:
     url: https://deb.nodesource.com/node_16.x
 
 plugs:
-  system-gitconfig:
+  etc-gitconfig:
     interface: system-files
     read:
     - /etc/gitconfig
-  user-gitconfig:
+  gitconfig:
     interface: personal-files
     read:
     - $HOME/.gitconfig