chore: delete unnecessary inheritance of FlowableBufferTimedFlushable

[OrezzerO]

Closes #

Proposed Changes

FlowableBufferTimedFlushable doesn't need to inherit from FlowableTransformer. Therefore, delete this inheritance relationship to reduce the complexity of the code. The original implementation was quite confusing.

Checklist

• [x] CHANGELOG.md updated
• [x] Rebased/mergeable
• [x] A test has been added if appropriate
• [x] mvn test completes successfully
• [x] Commit messages are conventional
• [x] Sign CLA (if not already signed)

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.51%. Comparing base (f9283d6) to head (50589b1). Report is 9 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #794      +/-   ##
============================================
+ Coverage     88.50%   88.51%   +0.01%     
+ Complexity      787      785       -2     
============================================
  Files           174      174              
  Lines          7203     7202       -1     
  Branches        405      405              
============================================
  Hits           6375     6375              
+ Misses          697      696       -1     
  Partials        131      131              

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[OrezzerO]

Which part of CHANGELOG.md should I record this PR ?

[bednar]

@OrezzerO, please add a new section titled “Features” to the 7.3.0 release notes. Thank you! 😊

[OrezzerO]

@bednar CHANGELOG.md updated.