refactor: upgrade Go-Akt to 2.6.0 and remove unnecessary code

[Tochemey]

closes #84

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 9 lines in your changes missing coverage. Please review.

Project coverage is 76.26%. Comparing base (9d13afb) to head (55ff8ce).
Report is 1 commits behind head on main.

Files	Patch %	Lines
projection/runner.go	57.14%	1 Missing and 5 partials ⚠️
engine.go	60.00%	0 Missing and 2 partials ⚠️
eventstore/memory/memory.go	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   77.18%   76.26%   -0.92%     
==========================================
  Files          20       20              
  Lines        1578     1500      -78     
==========================================
- Hits         1218     1144      -74     
+ Misses        231      227       -4     
  Partials      129      129              

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