FPHSA: Always populate the ExactCase_FOUND variable (#15412).

The UPPERCASE name was inconsistent with config-packages, the
find_dependency macro, and even FPHSA itself, which expects
components to be specified with names matching ExactCase.

The FOUND_VAR was only permitted to have two possible values, and
now both are set for compatibility.  Document it as obsolete, and
adjust the code for the same.  Users of the variable should just
remove it.

Help/release/dev/FPHSA-ExactCase-name.rst

@@ -0,0 +1,10 @@
+FPHSA-ExactCase-name
+--------------------
+
+* The :module:`FindPackageHandleStandardArgs` module
+  ``FIND_PACKAGE_HANDLE_STANDARD_ARGS`` function now
+  always populates the both ``<PackageName>_FOUND``
+  and ``<UPPERCASE_NAME>_FOUND`` variables (the latter
+  for backwards compatibility).  The ``FOUND_VAR``
+  option is now ignored except to enforce its allowed
+  values.