This commit updates the defaultResponseCharacterEncoding() `default`
method in ConfigurableMockMvcBuilder so that it throws an
UnsupportedOperationException instead of silently ignoring the user's
request to set the default response character encoding.
Note, however, that AbstractMockMvcBuilder already overrides the
default method with a concrete implementation which is used by default
in MockMvc.
See gh-27230
87d3d1bbd1