Update core 1.6 release docs to point to the new semantic layer guide instead of a PR

[graciegoheen]

Contributions

• I have read the contribution docs, and understand what's expected of me.

Link to the page on docs.getdbt.com requiring updates

https://docs.getdbt.com/guides/migration/versions/upgrading-to-v1.6#:~:text=As%20part%20of%20the%20Semantic%20layer%20re%2Dlaunch%20(in%20beta)%2C%20the%20spec%20for%20metrics%20has%20changed%20significantly.%20Migration%20guide%20coming%20soon%3A%20https%3A//github.com/dbt%2Dlabs/docs.getdbt.com/pull/3705

What part(s) of the page would you like to see updated?

Update this:

As part of the Semantic layer re-launch (in beta), the spec for metrics has changed significantly. Migration guide coming soon: #3705

To point to the new migration guide https://docs.getdbt.com/guides/migration/sl-migration instead of the PR

Additional information

No response