Rename "message data" to "message payload"

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	✔️
Tests pass?	✔️

[codecov]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (fd6d760) to head (c83165d).

Files with missing lines	Patch %	Lines
src/Message/Envelope.php	0.00%	4 Missing ⚠️
src/Message/GenericMessage.php	0.00%	4 Missing ⚠️
src/Message/Serializer/MessageSerializer.php	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             master    #311   +/-   ##
========================================
  Coverage      0.00%   0.00%           
  Complexity      334     334           
========================================
  Files            51      51           
  Lines           918     918           
========================================
  Misses          918     918           

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