Prepare v5 release series

[aldas]

We need to retract v0.50.0 which is for Echo 5 but this messes up people that still use v4 as they get it when they do go get -u ./...

So from now Echo v5 gets v5 major release number in this repository.

Relates to #142

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.69%. Comparing base (1e69e54) to head (5e4c5f2).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   63.77%   67.69%   +3.91%     
==========================================
  Files           9        9              
  Lines         969      585     -384     
==========================================
- Hits          618      396     -222     
+ Misses        308      157     -151     
+ Partials       43       32      -11     

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