Add CHANGELOG entries for 0.1.9 and 0.2.1

[TheBlueMatt]

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[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.10%. Comparing base (6447d98) to head (f42b9f6).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4364      +/-   ##
==========================================
+ Coverage   86.09%   86.10%   +0.01%     
==========================================
  Files         156      156              
  Lines      102800   102800              
  Branches   102800   102800              
==========================================
+ Hits        88504    88516      +12     
+ Misses      11786    11775      -11     
+ Partials     2510     2509       -1     

Flag	Coverage Δ
tests	86.10% <ø> (+0.01%)	⬆️

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[ldk-reviews-bot]

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[TheBlueMatt]

Definitely doesn't need a second reviewer.