CMakeLists: rename version to VERSION to avoid shadowing C++20 <version> - #570

Problem
-------
The file named `version` in the repository root contains the plain-text
project version string ("1.1.0") and is read by CMakeLists.txt at
configure time:

    file(READ version PROJECT_VERSION)

When rpi_ws281x is used as a CMake subdirectory of a C++20 project, the
subdirectory's source path is typically added to the compiler include
search path so that the library headers (ws2811.h, rpihw.h, etc.) can be
found with angle-bracket includes.  On any compiler that implements C++20,
the `<version>` feature-test header is part of the standard library.  When
a translation unit (directly, or through a transitively included header)
does:

    #include <version>

the preprocessor searches the include path in order.  If the rpi_ws281x
source directory appears on that path before the toolchain's system include
directory, the compiler opens the plain-text `version` file instead of the
C++20 standard library header.  Attempting to parse "1.1.0" as C++ source
is a hard error:

    error: stray '.' in program

Fix
---
Rename `version` to `VERSION`.  On Linux (and all other case-sensitive
filesystems) `VERSION` and `version` are distinct names; the compiler
looking for `<version>` will no longer find the version data file.

Update the single `file(READ ...)` call in CMakeLists.txt accordingly.
No other file in this repository references the plain name `version` as a
path: `version.h.in` and `version.py` are separate files with different
names and are unaffected.

Impact
------
Behaviour is identical for all existing build methods (CMake, SCons).  The
only observable change is the filename on disk; the content and the
PROJECT_VERSION variable it populates are unchanged.