Fix start trial button from being truncated

What does this MR do and why?

The code change removes the class="gl-w-20" attribute from the gl-button component. This means that the button will no longer have a fixed width of 20 characters. Instead, the button will automatically adjust its width to fit its content.

Relates to #465566 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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.

7. Select Start a free GitLab Duo Pro trial

8. Review the page for the updated primary button