Drop beta tag on Duo Pro promotion

What does this MR do and why?

The change replaces "GitLab Duo Chat (Beta)" with "GitLab Duo Chat" in a promotional message for the GitLab Duo Pro add-on. It also updates the translation for this message.

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Start the GDK in SaaS mode. Typically, this is done by setting the GITLAB_SIMULATE_SAAS environment variable:

   gdk stop
   export GITLAB_SIMULATE_SAAS=1
   gdk start

2. Enable the check_namespace_plan application setting:

   echo "ApplicationSetting.first.update(check_namespace_plan: true)" | rails c

3. Create a new top-level group.

4. Start a trial for the group you've just created in the rails console:

   group = Group.last
   ultimate_plan = Plan.find_by_name("ultimate_trial")
   trial_start_date = -5.days.from_now
   trial_end_date = trial_start_date + 30.days
   
   gitlab_subscription = group.gitlab_subscription
   gitlab_subscription.hosted_plan_id = ultimate_plan.id
   gitlab_subscription.trial = true
   gitlab_subscription.trial_starts_on = trial_start_date
   gitlab_subscription.trial_ends_on = trial_end_date
   gitlab_subscription.save!
   
   group.web_url # Prints the group's URL for convenience

5. Navigate to the group and notice the trial status widget in the left sidebar.

6. Navigate to -/billings and scroll down to find the promotion for the GitLab Duo Pro add-on.