Update Slack user group for a schedule - handle paid_team_only Slack API error

[joeyorlando]

What this PR does

Semi-related to https://github.com/grafana/oncall-private/issues/2131

Addresses occasional task failures for apps.slack.tasks.update_slack_user_group_for_schedules when trying to update a Slack user group for a non-paid Slack account. Slack's documentation mentions this is a paid only feature, hence the error (logs from an actual task):

2024-08-08 16:20:36,613 source=engine:celery worker=ForkPoolWorker-16 task_id=6bdaae94-1552-4b6d-93e2-e2fa0bae57b1 task_name=apps.slack.tasks.update_slack_user_group_for_schedules name=apps.slack.models.slack_usergroup level=WARNING Slack usergroup S06LW5GJ88Z update failed: Slack API error! Response: {'ok': False, 'error': 'paid_teams_only'}

Updated our docs on our Slack integration to emphasize that this feature only works for paid Slack accounts

Checklist

• Unit, integration, and e2e (if applicable) tests updated
• Documentation added (or pr:no public docs PR label added if not required)
• Added the relevant release notes label (see labels prefixed w/ release:). These labels dictate how your PR will
  show up in the autogenerated release notes.