Update migration guide

[discdiver]

First check

• [X] I added a descriptive title to this issue.
• [X] I used the GitHub search to find a similar request and didn't find it.
• [X] I searched the Prefect documentation for this feature.

Prefect Version

2.x

Describe the current behavior

A number of the Discourse links and content for v1 to v2 migration could use updated.

Describe the proposed behavior

Updated links/content.

Example Use

No response

Additional context

No response