Allow self-managed instances to require licensed seats for Duo Chat

What does this MR do and why?

This MR introduces the ability for self-managed GitLab instances to require licensed seats for Duo Chat using the existing duo_chat_requires_licensed_seat_sm feature flag. Before this MR, it was enabled globally due to Chat GA.

Here is a similar MR for .com. The difference is: with this MR, it is configured only by an instance-wide feature flag, whereas in .com, it can also be turned on or off per group.

Related to #457283 (closed)

MR acceptance checklist

✅

Screenshots or screen recordings

N/A

How to set up and validate locally

################
# Condition:
# Make sure user does not have Duo Pro add-on enabled
User.first.duo_pro_add_on_available? # => false

# Test Case: When flag is disabled, user can access chat.
Feature.disable(:duo_chat_requires_licensed_seat_sm)

# Test Case: When flag is enabled, user cannot access chat.
Feature.enable(:duo_chat_requires_licensed_seat_sm)


################
# Update condition:
# - The user has duo pro seat.
User.first.duo_pro_add_on_available? # => true

# Test Case: When flag is disabled, user can access chat.
Feature.disable(:duo_chat_requires_licensed_seat_sm)

# Test Case: When flag is enabled, user can access chat.
Feature.enable(:duo_chat_requires_licensed_seat_sm)