Preserve standaloneusers schema version and purge standalonemigrate in core pipeline

ufundorequested to merge
extension-juggling into develop
  • previously we had the standaloneusers schema version hardcoded -- this checks the value from the target site prior to migration (as this should correspond to the table structures that are in place)
  • make purging standalonemigrate from the civicrm_extension table a core pipeline step

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