Fix: Correct version strings for simdutf and cimgui to match vendored files

[AnthonyZhOon]

The cimgui version listed does not match the upstream commit or the vendored cimgui files
Checking the upstream git log the commit corresponds to commit e391fe2e66eb1c96b1624ae8444dc64c23146ef4 (tag: v1.90.6-docking) however the build.zig.zon is outdated.
The vendored cimgui files also contain the header

// This file is automatically generated by generator.lua from
// https://github.com/cimgui/cimgui based on imgui.h file version "1.90.6" 19060
// from Dear ImGui https://github.com/ocornut/imgui with imgui_internal.h api
// docking branch

I wasn't too clear with what the comment meant:

// This should be kept in sync with the submodule in the cimgui source
// code to be safe that they're compatible.

and assumed it was referring to the vendored cimgui files, added a comment pointing out where to find the cimgui source mentioned.

[AnthonyZhOon]

Also noticed simdutf had the wrong version compared to the vendored file

#define SIMDUTF_VERSION "5.2.8"

in simdutf.h
Is there a reason why a url isn't provided for simdutf?

[mitchellh]

Is there a reason why a url isn't provided for simdutf?

There is... I think we modify it to fix some Zig issues.