[luca-heltai]

Update pi-BEM's required deal.II version, Docker image, CI, helper scripts, examples and VERSION to 9.7.1/Noble. Replace deprecated SmartPointer names with ObserverPointer and clarify API documentation.

Two runtime issues are fixed:

• The inherited test harness enabled floating-point exceptions without restoring them at shutdown, causing ARM64 Debug tests to abort in CGAL. Mirror upstream deal.II commit 5b0d034e in the test helper's destructor.
• Direct real/complex preconditioners read boundary masks and Robin coefficients for remote columns from owned-only vectors. Collect the column data before rank-local assembly. A four-DoF MPI regression checks both matrices against explicit expected entries and reproduces the remote-read assertion with the previous library.

The obsolete Travis-based Docker runner now runs the current Debug/Release checks.

Validation with dealii/dealii:v9.7.1-noble on ARM64:

• Debug and Release builds with -Werror.
• 24/24 tests passed (26 CTest entries including two executable-build dependencies), including the two-rank MPI regression.
• All 12 real/complex DN/DR/NR Direct/FMA samples completed in both Debug and Release with two MPI ranks and two threads.
• Doxygen API generation completed without documentation warnings in an isolated output directory; shell syntax and indentation checked.

CI must pass on this PR's final commit before merge and release.