Update dynamic core parameter documentation

[Francyrad]

This PR addresses two small dynamic core parameter cleanups.

Changes:

• correct the Tm1 and Tm2 defaults and documentation to use SI units (1/Pa and 1/Pa^2) instead of terapascal-based units
• remove the unused Gravity acceleration parameter from the dynamic core input file interface and test input
• clarify that the adiabatic correction for the CMB heat flux uses the active Gravity model, so users should control CMB gravity through the Gravity model subsection

This does not add a multiplier or unit conversion in the code. It makes the documented/default values match the units already used by the formulas.

Validation:

• cmake --build build-debug --target aspect -j20
• ASPECT_SOURCE_DIR=/home/francyrad/Documenti/aspect_debug/aspect_issue_6926_work ctest --output-on-failure -R "dynamic_core|dynamic_core_fully_molten|dynamic_core_fully_solid"

Results on this x86_64 Debug setup:

• dynamic_core passes
• dynamic_core_fully_solid passes
• dynamic_core_fully_molten still reports the known last-digit statistics diff on this setup

Refs #6227.
Refs #6926.

[alarshi]

Thank you for updating the documentation and testing the changes locally! It looks good to me.
The documentation files (.md) are generated automatically from the corresponding parameter declarations in the source files, so for the next PRs, you could also just make the changes in the .cc files :)

[alarshi]

/rebuild