Revert back to using the default SPI build version for documentation builds

[daveverwer]

We recently switched SPI to use 6.0 by default. There would be no harm in leaving this as explicitly set to 6.0 for now, but while I remember it’s still here I thought I’d revert it so that when we get to 6.5, this explicitly set version hasn’t rolled off the bottom of what we support.

[fpseverino]

Wonderful! Thanks a lot

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.91%. Comparing base (8b0fbc9) to head (f8e283a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #6   +/-   ##
=======================================
  Coverage   88.91%   88.91%           
=======================================
  Files           8        8           
  Lines         523      523           
=======================================
  Hits          465      465           
  Misses         58       58           

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