Update 1.8 changelog to include reference to 1.7 changelog

[AartBluestoke]

resolves # docs dbt-labs/docs.getdbt.com/#

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• [x] I have read the contributing guide and understand what's expected of me
• [x] I have run this code in development and it appears to resolve the stated issue
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

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[AartBluestoke]

I've now signed the CLA