Refactor PrioritiesSelect to use modern Select component

Issue Description

The PrioritiesSelect component in apps/meteor/client/views/omnichannel/additionalForms/PrioritiesSelect.tsx currently uses the deprecated SelectLegacy component from @rocket.chat/fuselage.

It should be refactored to use the modern Select component, aligning it with other components in the codebase (such as SlaPoliciesSelect.tsx in the same directory).

Current Behavior

• Uses SelectLegacy.
• Relies on custom props like renderOptions, renderSelected, and renderItem, which are part of the legacy API.

Expected Behavior

• Refactor to use the Select component.
• Simplify the implementation by using standard SelectOption[] props.
• Remove reliance on deprecated legacy components.
• Improve accessibility (e.g., ensuring proper aria-labelledby usage).

Affected Files

• apps/meteor/client/views/omnichannel/additionalForms/PrioritiesSelect.tsx

Start with apps/meteor/client/views/omnichannel/additionalForms/PrioritiesSelect.tsx and compare it with SlaPoliciesSelect.tsx in the same directory. Replace the legacy SelectLegacy usage with the modern Select and standard SelectOption[] props, including proper aria-labelledby usage; done means the file no longer relies on renderOptions, renderSelected, or renderItem.