GPUUploadManager: rename CopyBufferCallbackType to CopyStagingBufferC… #4179

Job	Run time
Linux -> Pre-checks	19s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	6m 49s
Linux -> Clang-x64, Release	13m 26s
Linux -> Clang-x64, Debug	13m 23s
Linux -> GCC-x64, Release	10m 34s
Linux -> GCC-x64, Debug	11m 59s
Linux -> Clang-NO_GLSLANG-x64, Debug	6m 45s
	1h 3m 15s

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