Modernize a small aspect of step-20/21

[bangerth]

#13109 simplifies the initialization of block matrices and vectors by passing a vector of block sizes to the overarching block object, rather than initializing the blocks individually and then calling .collect_sizes() on the overarching object.

step-20 and step-21 are the remaining hold-outs. They do things differently, and a bit more thought is required. The documentation also needs to be adjusted as there is a lengthy text block explaining the way it is done right now.

Separately, at least one of these two still uses non-dynamic sparsity patterns. We should drop this -- it's not efficient, and we should never do that anymore.