Update README with end-of-support timeline and add migration links

[jj22ee]

Issue #, if available:

Description of changes:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.28%. Comparing base (2b80b7b) to head (ad2d02c).
⚠️ Report is 26 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #721   +/-   ##
=======================================
  Coverage   84.28%   84.28%           
=======================================
  Files          36       36           
  Lines        1814     1814           
=======================================
  Hits         1529     1529           
  Misses        285      285           

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