Fix missing cut plane in CutMeshByPlaneGenerator

[miaoyinb]

closes #26902

[miaoyinb]

@GiudGiud A minor bug fix. Thanks in advance.

[GiudGiud]

Test failures are unrelated btw. work on moose happening

[GiudGiud]

this one is not btw

In file included from /data/civet2/build/moose/framework/build/unity_src/meshgenerators_Unity.C:19:
/data/civet2/build/moose/framework/src/meshgenerators/CutMeshByPlaneGenerator.C:115:20: error: variable 'elem_vertices_counter_1' set but not used [-Werror,-Wunused-but-set-variable]
    unsigned short elem_vertices_counter_1(0);
                   ^

[moosebuild]

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[moosebuild]

Job Coverage on afa2585 wanted to post the following:

Framework coverage

	d6b47a	#26903 afa258
	Total	Total	+/-	New
Rate	85.29%	85.29%	+0.00%	100.00%
Hits	100795	100793	-2	4
Misses	17385	17384	-1	0

Diff coverage report

Full coverage report

Modules coverage

Coverage did not change

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Reports

• framework
• chemical_reactions
• combined
• contact
• electromagnetics
• external_petsc_solver
• fluid_properties
• fsi
• functional_expansion_tools
• geochemistry
• heat_transfer
• level_set
• misc
• navier_stokes
• optimization
• peridynamics
• phase_field
• porous_flow
• ray_tracing
• rdg
• reactor
• richards
• scalar_transport
• solid_properties
• stochastic_tools
• tensor_mechanics
• thermal_hydraulics
• xfem

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