Make backlight timing a float

[DominicOram]

Needed to help typing in DiamondLightSource/mx-bluesky#560

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 95.02%. Comparing base (c2096b6) to head (fed070a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #831   +/-   ##
=======================================
  Coverage   95.02%   95.02%           
=======================================
  Files         117      117           
  Lines        4765     4765           
=======================================
  Hits         4528     4528           
  Misses        237      237           

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