move accelerator_connector.py to the connectors subfolder

[awaelchli]

What does this PR do?

Follow up to #5743
Moves the accelerator connector to the folder where all other connectors are.
This has no implications for the user, as this class is internal.

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[codecov]

Codecov Report

Merging #6033 (4813dc6) into master (5157ba5) will decrease coverage by 2%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #6033    +/-   ##
=======================================
- Coverage      93%     91%    -2%     
=======================================
  Files         160     160            
  Lines       11340   11342     +2     
=======================================
- Hits        10550   10282   -268     
- Misses        790    1060   +270     

[tchaton]

LGTM !