Deprecate h_item

[KornevNikita]

Describe the bug

SYCL 2020 has deprecated h_item, but the compiler still doesn't issue a warning when using h_item. Need to deprecate this class (but do not remove).

To reproduce

1. Include a code snippet that is as short as possible
2. Specify the command which should be used to compile the program
3. Specify the command which should be used to launch the program
4. Indicate what is wrong and what was expected

Environment

• OS: [e.g Windows/Linux]
• Target device and vendor: [e.g. Intel GPU]
• DPC++ version: [e.g. commit hash or output of clang++ --version]
• Dependencies version: [e.g. the output of sycl-ls --verbose]

Additional context

No response