dev/core#2139 fix defaulting for multi-select custom fields

[alifrumin]

Overview

Fixes editing defaults for multi-select custom fields so that:

1. One can select multiple options to be defaulted
2. The default shows up on backend forms correctly (before there was a bug where on backend forms where edited multi-select custom fields were not displaying the defaults)

See https://lab.civicrm.org/dev/core/-/issues/2139 for more details.

Before

1. Create a new Multi-Select Field
   
2. enter a few options and set one to be the default.
3. Save the new field
4. Edit the field options for this field to have a different default.
5. Go to a form that displays that field on the backend (ex: if the field is on individuals, go to the "New Individual" Form).

EXPECTED RESULT
The field will not show anything defaulted.

After

The default should show correctly (and the user should be able to select multiple options to be defaults)

[civibot]

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[colemanw]

Thank you so much for the fix @alifrumin!
I reproduced the bug & agree it is a regression.
I've tested this with both single & multi-select fields and both are working correctly thanks to your patch!