Update overlay_channel for i19-1

[noemifrisina]

Can't use channel 1 or 2 on the overlay, so update the channel for the pin tip centre holder

Part of i19-bluesky#116

Instructions to reviewer on how to test:

1. Do thing x
2. Confirm thing y happens

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.08%. Comparing base (98eb4b6) to head (45ebc4b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1869   +/-   ##
=======================================
  Coverage   99.08%   99.08%           
=======================================
  Files         299      299           
  Lines       11323    11323           
=======================================
  Hits        11219    11219           
  Misses        104      104           

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[DominicOram]

Great, thank you!