Should `sycl::exception::what()` be noexcept?

### Specification Version

SYCL 2020 (Revision 11)

### Section Number(s)

4.13.2

### Bug Description

[std::exception](https://en.cppreference.com/cpp/error/exception/what) declares `what()` as:
```
virtual const char* what() const noexcept;
```
`sycl::exception` inherits `std::exception`, therefore `sycl::exception::what()` should also be `noexcept`, otherwise it causes weaker exception specification.

Start with SYCL 2020 Revision 11, section 4.13.2, and compare the documented sycl::exception::what() signature with std::exception::what(). Update the specification so the exception specification matches the inherited declaration, then check the surrounding section for consistent wording.

Written by the indexing model from the issue text.