Various mixture-related fluid properties improvements

[joshuahansel]

This PR does the following, in support of the upcoming THM gas mixture flow channel:

• Added default implementations for SinglePhaseFluidProperties::criticalDensity() and criticalInternalEnergy() by evaluating respective methods with criticalPressure() and criticalTemperature().
• Closes Fluid properties interrogator does not report invalid properties combination for corner case #29656
• Stopped outputting vapor mixture properties twice in FluidPropertiesInterrogator
• Added debug information to the solve-failed exception in BrentsMethod (iteration history)
• Added REL_ABS_TEST macro to FluidPropertiesTestUtils
• Added s(p,T) to VaporMixtureFluidProperties and interrogator
• Went nuts on cleaning up IdealRealGasMixtureFluidProperties and its Test
• Added warnings in IdealRealGasMixtureFluidProperties where derivatives have been neglected
• Added methods to IdealRealGasMixtureFluidProperties for getting the primary and secondary fluid properties objects

[moosebuild]

Job Documentation, step Docs: sync website on ba794d0 wanted to post the following:

View the site here

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