Resolve duplicate content on updating message templages

Created by: seancolsen

These two sections have very similar content

https://docs.civicrm.org/sysadmin/en/latest/upgrade/#updating-message-templates https://docs.civicrm.org/user/en/latest/email/message-templates/#upgrades-and-customized-system-workflow-templates

Resolve by:

1. Choosing one place for the content to live (I don't have an opinion about this)
2. Merging the content into one body of text that preserves concepts from both sources
3. Provide links that point readers to the new content from wherever it was removed