[SYCL][DOC] Move old spec constant extension spec (#5456)

This old specialization constant extension is still supported, but it
is superseded by the specialization constant APIs in the core SYCL 2020
specification.  It's implementation is in the "experimental" namespace,
so move it to the "experimental" directory and add a note that it is
now deprecated.

Author: gmlueck

sycl/doc/extensions/SpecConstants/README.md renamed to sycl/doc/extensions/experimental/SYCL_EXT_ONEAPI_SPEC_CONSTANTS.md

@@ -11,8 +11,10 @@ constants based on the
 [proposal](https://github.com/codeplaysoftware/standards-proposals/blob/master/spec-constant/index.md)
 from Codeplay.
 
-**NOTE:** _In future versions it may be superseded by [SYCL 2020
-specification](https://www.khronos.org/registry/SYCL/specs/sycl-2020-provisional.pdf)._
+**NOTE:** This extension is now deprecated.  Use the core SYCL specialization
+constant APIs defined in the
+[SYCL 2020 specification](https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html)
+instead.
 
 A specialization constant is identified by a C++ type name, similarly to a
 kernel, its value is set via `program::set_spec_constant` class API and is