[REF] Cleanup customField prepareCreate function

[colemanw]

Overview

Followup from #14689 - this does some additional code cleanup to the extracted function. Should have no functional impact.

Before

Messier. Uses some older deprecated methods of fetching settings & serializing fields.

After

Neater. Updated methods.

[civibot]

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[colemanw]

retest this please

[eileenmcnaughton]

I worked through the cleanup here & agree it is sensible & should have no functional impact. Let's see if the test run still fails though