Back compat for previous return type of collect_freshness

[jtcohen6]

resolves #7489

Background:

• Previously, the built-in collect_freshness macro (used by dbt source freshness) returned just an agate.Table of values.
• In v1.5, we updated it to return the full query result, including the associated adapter_response (metadata)

Changes in this PR:

• Check the return type before unpacking in adapter.calculate_freshness
• Raise a deprecation warning if the calculate_freshness macro returns only an agate.Table

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR - How to update custom overrides of collect_freshness macro for 1.5 docs.getdbt.com#3331
• I have run changie new to create a changelog entry

[aranke]

Mostly LGTM, few nits

[aranke]

LGTM!

[github-actions]

The backport to 1.5.latest failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-1.5.latest 1.5.latest
# Navigate to the new working tree
cd .worktrees/backport-1.5.latest
# Create a new branch
git switch --create backport-7535-to-1.5.latest
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 35f8ceb7f1dc575f1fc2e9e573e526f64cf25c9f
# Push it to GitHub
git push --set-upstream origin backport-7535-to-1.5.latest
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-1.5.latest

Then, create a pull request where the base branch is 1.5.latest and the compare/head branch is backport-7535-to-1.5.latest.