snap: Use interface names etc-gitconfig and gitconfig, Take 2

snap: Use interface names etc-gitconfig and gitconfig, Take 2

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

I neglected to make the rename of system-gitconfig → etc-gitconfig and
user-gitconfig → gitconfig to the apps.plugs section in commit fd3953c,
hence this commit to fix the store upload scan failure:

  interface 'system-gitconfig' not found in base declaration
  interface 'user-gitconfig' not found in base declaration
  unknown plugs interface name reference 'system-gitconfig'
  unknown plugs interface name reference 'user-gitconfig'

Special thanks to Alex Murray and Igor Ljubuncic for approving the
system-files and personal-files request for Hugo.

Fixes #6226