Remove prohibited aria-label from HasMany badge

[Flo0807]

Problem

CI on #2206 (Update dependency a11y_audit to ~> 0.5.0) fails with one accessibility violation:

1) test posts edit a11y (DemoWeb.Browser.PostBrowserTest)
   Expected page to have no accessibility violations, but got 1 violation.

   serious  Elements must only use permitted ARIA attributes
   https://dequeuniversity.com/rules/axe/4.13/aria-prohibited-attr

   There are 2 nodes with this violation:
   1. <label class="flex cursor-pointer items-center pr-2" for="has-many-tags-checkbox-value-…
        - aria-label attribute cannot be used on a label with no valid role attribute.

a11y_audit 0.5.0 ships a newer axe-core, whose aria-prohibited-attr rule now flags this. The <label> element has no corresponding ARIA role, so aria-label is not allowed on it — and is ignored by assistive technology anyway.

Fix

The offending element is the "x" of a selected badge in Backpex.Fields.HasMany. It is a <label for=…> pointing at the option's checkbox, so clicking it unselects the option.

That label is a pointer-only shortcut: labels are not focusable, so keyboard and screen reader users never reached it. They unselect an option via the checkbox in the dropdown menu instead. So the label is now hidden from assistive technology with aria-hidden="true" rather than given a name it cannot expose.

This has a second benefit: the badges live inside the dropdown trigger, which has role="button" and therefore derives its accessible name from its contents. Previously each badge contributed a duplicated "Unselect " to that name; now the trigger announces just the selected labels.

Alternatives considered:

• sr-only text inside the label — the label is associated with the checkbox via for, so the text would be appended to the checkbox's accessible name ("Tag Unselect Tag") and to the trigger's.
• role="button" on the label (as multi_select_badge in Backpex.HTML.Form does on a <div>) — would permit aria-label, but creates a button that is not keyboard focusable, and risks tripping aria-allowed-role.

The "Unselect %{label}" translation is still used by Backpex.HTML.Form.multi_select_badge, where it sits on a <div role="button"> and is permitted, so no gettext changes are needed.

Verification

Run locally against a11y_audit 0.5.0 (the version from #2206):

• Before: Result: 3/4 passed — posts edit a11y fails with the violation above.
• After: Result: 41 passed, 325 excluded (full --only playwright suite; previously 40/41).

Also green: mix test (158 doctests, 447 tests, 0 failures), mix format, mix credo.

Once this lands, #2206 should go green after a rebase.