Fix inconsistent ufl usage in demos (#3532) #33372

Job	Run time
Lint	16s
fenicsx-refs / Set FEniCSx component git refs	1s
Build and test	19m 37s
Build and test (linux-gnu-real32-32, ghcr.io/fenics/test-env:current-openmpi)	10m 22s
Build and publish docs	5m 37s
Build and test (linux-gnu-real64-32, ghcr.io/fenics/test-env:current-openmpi)	10m 45s
Build and test (linux-gnu-complex64-32, ghcr.io/fenics/test-env:current-openmpi)	13m 33s
Build and test (linux-gnu-complex128-32, ghcr.io/fenics/test-env:current-openmpi)	14m 7s
Build and test (linux-gnu-real64-64, ghcr.io/fenics/test-env:current-openmpi)	10m 46s
Build and test (linux-gnu-complex128-64, ghcr.io/fenics/test-env:current-openmpi)	14m 7s
Build and test (linux-gnu-real64-32, ghcr.io/fenics/test-env:current-mpich)	10m 58s
Build and test (linux-gnu-complex128-32, ghcr.io/fenics/test-env:current-mpich)	13m 49s
	2h 3m 58s

Provide feedback