DOCS: Imports in introductory tutorials not up to date (imports deprecated `VTKFile`)

[p479h]

These examples:

• ear wave equation with optional mass lumping.
• A basic Helmholtz equation.
• The Burgers equation, a non-linear, unsteady example.
• A mixed formulation of the Poisson equation.

Have this line:
from firedrake.output import VTKFile
which does not work since I last updated the library (2 weeks ago).

I believe the solution is to simply remove this line and replace instances of VTKFile with File.

[connorjward]

We changed File to VTKFile last week. File is the deprecated name! You should update your Firedrake and then hopefully things should work for you.