Change the base C++ version

[gmlueck]

Change wording to allow the core C++ language to be any version C++17 or later.

Move sections about normative references and non-normative wording to the introduction, so they are earlier in the document.

My longer term goal is to remove "chapter/references.adoc" and add any references we require to the "Normative references" section. Some more work needs to be done for this to happen:

• Decide if we need to list the C++ defect reports as normative references. Currently, the "references" appendix lists only DR2325. Is there a reason to list this one and not others?

• Decide if OpenCL should be a normative reference. If so, which version(s)?

[keryell]

Thanks!

[etomzak]

How much confidence is there that rebasing SYCL onto newer C++ standards doesn't (drastically) change the meaning of (too many) things in the SYCL spec, as noted here for example?

[Pennycook]

How much confidence is there that rebasing SYCL onto newer C++ standards doesn't (drastically) change the meaning of (too many) things in the SYCL spec, as noted here for example?

I forget where it was discussed, but I think this is a known risk at this point.

There are some other terms that changed in C++20 (e.g., related to the memory model) and we'll need to make sure that we remain consistent with those changes. There is a dedicated sub-committee for alignment with ISO C++, so if we encounter issues like this I think we're prepared to do the work to fix things up.

[tomdeakin]

WG proved to merge