Upgrade externals September 2026

[tom-osika]

Follow Semi-automated monthly upgrades documentation.

Prior Issue (August): #24787

• [workspace] Update dependency abseil_cpp_internal to 20260817.0 #24954
• [workspace] Update crate_universe to latest #24952
• Upgrades September 2026 #24951
• [workspace] Update dependency vtk_internal to latest commit #24915
• Skipping mpmath_py_internal until sympy is updated to 1.15 (see [workspace] Update dependency mpmath_py_internal to 1.4.0 #24179)
• Skipping qdldl_internal v0.1.9, OSQP 1.0.0 is pinned to v0.1.8
• [setup] Upgrade Python venv to latest #24955
• [workspace] Update dependency vtk_internal to latest commit #24956

[tom-osika]

Unless otherwise preferred, I'll close this now and separate out the doxygen upgrade into a new issue.