customer feedback updates

[fivetran-joemarkiewicz]

Updates to the dbt_intercom_source package include:

• Intercom ERD README reference update.
• Removal of unnecessary columns through ALL staging models.
• Addition of plan fields to stg_intercom__company_history.
• Addition of sla fields to stg_intercom__conversation_history.
• Renaming certain fields within stg_intercom__company_history and stg_intercom__contact_history to accommodate upstream model changes.
• .yml updates to reflect above mention column additions and name changes.