[SYCL][DOC] Fix links to old SubGroupMask doc (#5459)

These links were always broken because the extension used to be
called "SubGroupMask", not "GroupMask".  Fix these links, pointing to
the new location of that extension after the rename in
2521592.

Author: gmlueck

sycl/ReleaseNotes.md

@@ -24,7 +24,7 @@ Release notes for commit range 4fc5ebe..bd68232
    [88cfe16]
  - Added [SYCL_INTEL_bf16_conversion extension document](doc/extensions/experimental/SYCL_EXT_INTEL_BF16_CONVERSION.asciidoc)
    [9f8cc3af]
- - Align [SYCL_EXT_ONEAPI_GROUP_MASK extension](doc/extensions/GroupMask/GroupMask.asciidoc)
+ - Align [SYCL_EXT_ONEAPI_GROUP_MASK extension](doc/extensions/supported/SYCL_EXT_ONEAPI_SUB_GROUP_MASK.asciidoc)
    with SYCL 2020 specification [a06bd1fb]
  - Added [documentation](doc/SYCLInstrumentationUsingXPTI.md) of XPTI related
    tracing in SYCL [1308fe7b]
@@ -1830,7 +1830,7 @@ Release notes for the commit range e8f1f29..ba404be
     [SubGroupAlgorithms](doc/extensions/SubGroupAlgorithms/SYCL_INTEL_sub_group_algorithms.asciidoc)
     [d9b178f]
   - Added extension introducing group masks and ballot functionality:
-    [GroupMask](doc/extensions/GroupMask/SYCL_INTEL_group_mask.asciidoc)
+    [GroupMask](doc/extensions/supported/SYCL_EXT_ONEAPI_SUB_GROUP_MASK.asciidoc)
     [d9b178f]
   - The project has been renamed to "oneAPI DPC++ Compiler", all documentation
     has been fixed accordingly [7a2e75e]

sycl/doc/extensions/README.md

@@ -8,7 +8,6 @@ DPC++ extensions status:
 |  Extension  |    Status   |   Comment   |
 |-------------|:------------|:------------|
 | [SYCL_INTEL_group_algorithms](GroupAlgorithms/SYCL_INTEL_group_algorithms.asciidoc)                                         | Deprecated                                | |
-| [GroupMask](GroupMask/GroupMask.asciidoc)                                                                                   | Proposal                                  | |
 | [Reductions for ND-Range Parallelism](Reduction/Reduction.md)                                                               | Partially supported(OpenCL: CPU, GPU; CUDA) | Not supported: multiple reduction vars, multi-dimensional reduction vars |
 | [SPV_INTEL_function_pointers](SPIRV/SPV_INTEL_function_pointers.asciidoc)                                                   | Supported(OpenCL: CPU, GPU; HOST)         | |
 | [SPV_INTEL_inline_assembly](SPIRV/SPV_INTEL_inline_assembly.asciidoc)                                                       | Supported(OpenCL: GPU)                    | |