Issue: #19715

Removing FindQt.cmake gives Qt upstream a path forward to export its
own QtConfig.cmake files which can be found by find_package()
without having to explicitly specify CONFIG. Projects that still
want to use Qt3/4 can call find_package(Qt[34]), include(FindQt),
or add FindQt.cmake to their CMAKE_MODULE_PATH.

Some modules assume that `/usr` and `/usr/local` are the only
`CMAKE_PREFIX_PATH` entries and explicitly enumerate all paths instead
of using `PATH_SUFFIXES` and relying on the dynamic set of prefix paths.
This commit attempts to rectify that behavior.

Per-source copyright/license notice headers that spell out copyright holder
names and years are hard to maintain and often out-of-date or plain wrong.
Precise contributor information is already maintained automatically by the
version control tool.  Ultimately it is the receiver of a file who is
responsible for determining its licensing status, and per-source notices are
merely a convenience.  Therefore it is simpler and more accurate for
each source to have a generic notice of the license name and references to
more detailed information on copyright holders and full license terms.

Our `Copyright.txt` file now contains a list of Contributors whose names
appeared source-level copyright notices.  It also references version control
history for more precise information.  Therefore we no longer need to spell
out the list of Contributors in each source file notice.

Replace CMake per-source copyright/license notice headers with a short
description of the license and links to `Copyright.txt` and online information
available from "https://cmake.org/licensing".  The online URL also handles
cases of modules being copied out of our source into other projects, so we
can drop our notices about replacing links with full license text.

Run the `Utilities/Scripts/filter-notices.bash` script to perform the majority
of the replacements mechanically.  Manually fix up shebang lines and trailing
newlines in a few files.  Manually update the notices in a few files that the
script does not handle.

In case the given version was not only "3" or "4", but something like "4.8"
DESIRED_QT_VERSION was set to an unsupported value. While at it also check
that the version passed in is really in the range of 3.x and 4.x. Also
suggest switching to the more specific find modules if possible.

The MATCHES tests were actually wrong, as "a4b" and "42" would also cause a
match when it should not.