Link to latest IBC spec

[cwgoes]

Update the IBC specification in this repo to link to the ICS repo (leaving the spec as-is, I don't think we need to delete it, just be clear that it won't be canonical).

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #4022 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #4022   +/-   ##
========================================
  Coverage    60.01%   60.01%           
========================================
  Files          212      212           
  Lines        15104    15104           
========================================
  Hits          9064     9064           
  Misses        5419     5419           
  Partials       621      621