Take strain_free_density as initial value for density calculation

[tophmatthews]

Creates new MaterialPropertyName strain_free_density that will replace the now deprecated density param. This enables the use of a material property for density independent of strains (say for example, dependent on chemistry), which can be changed throughout a simulation. Added test for Density in modules/misc/test/tests as well since Density is in the misc module, but is only tested in combined (almost seems like density should be in solid_mechanics but whatevs).

ping maybe @bwspenc and @dschwen ?

Closes #29556

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Job Documentation, step Docs: sync website on 443539c wanted to post the following:

View the site here

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Job Coverage, step Generate coverage on 443539c wanted to post the following:

Framework coverage

Coverage did not change

Modules coverage

Misc

	32662f	#29773 443539
	Total	Total	+/-	New
Rate	60.92%	69.62%	+8.69%	91.67%
Hits	251	291	+40	11
Misses	161	127	-34	1

Diff coverage report

Full coverage report

Full coverage reports

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_mechanics
• solid_properties
• stochastic_tools
• thermal_hydraulics
• xfem

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[tophmatthews]

Ready for you to take a look @bwspenc thanks!