Add release notes for 1.20.3

[ikhoon]

No description provided.

[jrhee17]

Thanks @ikhoon 🚀 🚀 🚀

[minwoox]

👍 👍 👍

[codecov]

Codecov Report

Base: 74.07% // Head: 74.07% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (508970c) compared to base (ab20641).
Patch has no changes to coverable lines.

❗ Current head 508970c differs from pull request most recent head 28c448f. Consider uploading reports for the commit 28c448f to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4526      +/-   ##
============================================
- Coverage     74.07%   74.07%   -0.01%     
  Complexity    18189    18189              
============================================
  Files          1537     1537              
  Lines         67429    67429              
  Branches       8533     8533              
============================================
- Hits          49951    49945       -6     
- Misses        13408    13414       +6     
  Partials       4070     4070              

Impacted Files	Coverage Δ
...corp/armeria/common/stream/AggregationSupport.java	76.00% <0.00%> (-6.00%)	⬇️
...a/internal/client/dns/SearchDomainDnsResolver.java	87.14% <0.00%> (-5.72%)	⬇️
...eria/internal/common/stream/StreamMessageUtil.java	61.11% <0.00%> (-5.56%)	⬇️
...rmeria/internal/client/dns/CachingDnsResolver.java	81.08% <0.00%> (-5.41%)	⬇️
...m/linecorp/armeria/client/DecodedHttpResponse.java	85.00% <0.00%> (-5.00%)	⬇️
...rmeria/common/stream/ConcatArrayStreamMessage.java	83.14% <0.00%> (-2.25%)	⬇️
.../linecorp/armeria/client/Http2ResponseDecoder.java	68.53% <0.00%> (-2.10%)	⬇️
...p/armeria/common/stream/FuseableStreamMessage.java	83.85% <0.00%> (-1.87%)	⬇️
...a/internal/common/stream/ByteBufsDecoderInput.java	86.33% <0.00%> (-1.44%)	⬇️
...eria/internal/common/AbstractKeepAliveHandler.java	81.42% <0.00%> (-1.10%)	⬇️
... and 14 more

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[ikhoon]