[Spec change] Clarify and deprecate "execution_capabilities"

[sycl-issue-bot]

Please review whether KhronosGroup/SYCL-Docs#625 by @gmlueck requires any changes to the CTS.

If changes are required: Open a new PR addressing the changes and link it to this issue.
If no changes are required: Close this issue and proceed with the spec PR.

[gmlueck]

This PR will need some CTS testing. Let's wait for comments on that PR, though, before outlining a test plan.

[gmlueck]

The CTS currently calls device::get_info<info::device::execution_capabilities>() and checks the return type of the value that is returned. This test needs to be modified to include try / catch, so that the test doesn't fail if the implementation throws an exception when the backend is not backend::opencl.

Extra points if the CTS also verifies that an exception is thrown when the backend is not backend::opencl.