Closed vocabularies for the tag input type still allow custom values as long as there's a suggestion present #6332

Describe the bug

Even when a vocabulary is closed, if you use the tag input type for it, your input will always be appeneded to the list as a 'suggestion', allowing you to submit any value (as long as your input causes a suggestion to show).

To Reproduce

1. Set any field up in the back end's submission-forms.xml to use a vocabulary with closed=true and input type tag e.g:

                <field>
                    <dc-schema>dc</dc-schema>
                    <dc-element>subject</dc-element>
                    <dc-qualifier></dc-qualifier>
                    <repeatable>true</repeatable>
                    <label>Subject Keywords</label>
                    <input-type>tag</input-type>
                    <hint>Enter appropriate subject keywords or phrases.</hint>
                    <required></required>
                    <vocabulary closed="true">srsc</vocabulary>
                </field>

2. Start tying a value that appears in the vocabulary, but only partially so that your input is not in the vocabulary
3. Verify that your input is shown as suggestion at the bottom of the list, you can click it, it's added as a tag and you can save/submit

In the images below you can see how I add "tes" which is NOT in my vocabulary

Expected behavior

A clear and concise description of what you expected to happen.

I've already got a fix on DSpace 9, so I will claim this issue myself and re-do it against the latest main