Fix TagFilterField definition to make dynamic-groups happy

[glennmatthews]

Closes: #N/A

What's Changed

#4178 added a to_field_name = "slug" attribute on TagFilterField and logic in DynamicGroup.set_filter that expected all TagFilterField to have this attribute (TagFilterField is a MultipleChoiceField subclass, not a ModelMultipleChoiceField subclass, and so it doesn't natively have any to_field_name attribute). When we merged develop up into next (#4271) the to_field_name line was deleted instead of being updated to to_field_name = "name" as it should have been. However, the automated tests added in #4178 didn't catch this issue until #4284 fixed DeviceFilterForm to have a tags TagFilterField instead of the incorrectly-named-for-2.0 tag field.

Solution: add to_field_name = "name" to make dynamic-groups happy for now.

TODO

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