dev/core#1974: correct UI regression on custom field edit

[highfalutin]

Overview

Corrects the UI issue on the Custom Field edit form mentioned in https://lab.civicrm.org/dev/core/-/issues/1974

Before

When editing an existing Multi-Select field, the form showed the type as "Select" in a frozen element and, lower down on the form, offered a "Multi-Select" checkbox which could be unchecked, but doing so had no effect. The user might think they had changed the field from multi-select to select, but the change was not saved.

After

The form shows the type as "Select (Multi-Select)" in a static element. The user can still change the field type by using the "Change Input Field Type" link.

That's in Edit mode. In Create mode, the "Multi-Select" checkbox makes sense, so we keep it appearing (and working) as before.

Notes

"Select (Multi-Select)" is a bit awkward. I'd prefer just "Multi-Select" but this solution keeps the code changes to a minimum.

[civibot]

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[eileenmcnaughton]

Ok - I'm a little fuzzy on this but I'm happy that you have gotten your head fully into it since it's fairly edge / low risk