Add transient tests for boundary coupling freeflow-pnm and freeflow-pm

Closes #1401

See merge request !3872

without using the updated init function of the couplingmanager with prevSol, the code resulted in an assertion error when running in debug mode

Co-authored-by: Timo Koch <timokoch@uio.no>

[md][couplingmanager] Allow for r-value SolutionVectorStorage in attachSolution

See merge request !3876

The storage can be passed as l-value or r-value. The solution itself is still mutable.

[multidomain][couplingmanager] Remove unused implementation detail

See merge request !3875

This was forgotten in the cleanup for dumux 3.6 where it had been used for deprecating an interface.

[io][subgrid][yasp] Get all params when initializing

See merge request !3873

Fix/fcstaggered parallel

See merge request !3858

[readme] Update latest release pipeline to 3.9

See merge request !3871

[common] Add option for five-point stencil numeric difference

See merge request !3866

Move Changelog entry to correct release cycle

See merge request !3863

[gitignore] Ignore more Python-related clutter

See merge request !3859

[linear] Adapt istl solver factories to OperatorTraits

See merge request !3857

Fix .mailmap tk

See merge request !3855

[test][md][boundary][stokesdarcy][1p2c] remove double include

See merge request !3856

[cleanup] Remove unused variables flux

See merge request !3854

Found by Clang 18.

Feature/doc disc images

See merge request !3850