Primevue multiselect is deprecated

### Description

According to the [page on multiselect](https://primevue.dev/multiselect/) it's deprecated, and can be replaced by select with a `multiple` flag.

This isn't a high priority until they remove it, but it would be nice to make out single and multi select implementations more consistent.

Find the existing single- and multi-select implementations and compare how they use PrimeVue. Start with the current multiselect usage and the corresponding select implementation; replace the deprecated multiselect with select using the multiple flag, then verify that single- and multi-select behavior is consistent.

Written by the indexing model from the issue text.