FindDoxygen: Implement more complete version checking

The foundation of the Doxygen Find Module's detection methodology
is the command `find_program`, which has inhibited the module from
properly handling user version restrictions when provided.

Because `find_program` historically has always returned after the first
match and does not consider version constraints, users of this module
are inadvertently at the mercy of the command's search procedure.
Essentially, `find_package(Doxygen ...)` will always provide the first
Doxygen build found through said procedure, even if it conflicts with
the user's version requirements, and even if another build exists on
the system that would satisfy those requirements (i.e. shadowing).

Utilizes the new `VALIDATOR` option of `find_program` to ensure all
otherwise detectable builds of Doxygen on a given system are evaluated
and that only a build in compliance with `Doxygen_FIND_VERSION`
et. al., when defined, will be matched against.

Also enables handling of version ranges specified within `find_package`
via the **FindPackageHandleStandardArgs** module.

Fixes: #23692
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