message: Update CHANGELOG for ex28(p).
diff: Version 4.2.1 (development)
  ===========================
  +- Introduced solver interface for linear problems with constraints, a few
  + concrete solvers that implement the interface, and a demonstration of their
  + use in Example 28(p), which solves an elasticity problem with zero normal
  + displacement (but allowed tangential displacement) on two boundaries.
  +
  - Added high-order matrix-free auxiliary Maxwell solver for H(curl) problems,
  as described in Barker and Kolev 2020 (https://doi.org/10.1002/nla.2348). See
  Example 3p and linalg/auxiliary.?pp.
  