FindDoxygen: Improve search for Graphviz "dot" on Windows

As of Graphviz 2.31, their installer provides no PATH or registry
modifications.  Glob possible install paths instead.

Inspired-by: Lars Bilke <larsbilke83@googlemail.com>
FindDoxygen: Deprecate DOXYGEN_DOT_PATH

While at it, do not cache the value since it should always be
computed directly from DOXYGEN_DOT_EXECUTABLE.

Inspired-by: Lars Bilke <larsbilke83@googlemail.com>
Genex: Clarify error message on use with non-binary targets (#14899).
Merge topic 'consistent-genex-target-message' into next

a1d758e9 Genex: Clarify error message on use with non-binary targets (#14899).
Merge topic 'FindDoxygen-windows-dot' into next

f49b6a33 FindDoxygen: Deprecate DOXYGEN_DOT_PATH
7980eab8 FindDoxygen: Improve search for Graphviz "dot" on Windows