[DOC]: Update Memory Model and Execution Model to refer to C++26 release, once released

[gonzalobg]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this bug and that I agree to the Code of Conduct

Is this for new documentation, or an update to existing docs?

New

Describe the incorrect/future/missing documentation

The Memory Model docs haven't been updated to refer to C++23, and the Execution Model docs refer to the C++26 draft, because there are some relevant forward progress changes in C++26 already.

This issue tracks updating both to refer to the C++26 standard, once that's released.

[ ] We should also make sure the docs are properly readable.

If this is a correction, please provide a link to the incorrect documentation. If this is a new documentation request, please link to where you have looked.

No response