Make usage of vintage_and_active_years() in the tutorials consistent

[glatterf42]

Closes #897 and #899:

• For some tutorials, remove year_df entirely because it is unused. For others, make its usage consistent with westeros_baseline.
• Address a FutureWarning from pandas.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• [ ] Add, expand, or update documentation. Just fixing tutorials.
• [ ] Update release notes. Just fixing tutorials.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.6%. Comparing base (ab9b707) to head (0109580).
Report is 6 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #900   +/-   ##
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  Coverage   95.6%   95.6%           
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  Files         46      46           
  Lines       4338    4338           
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  Hits        4148    4148           
  Misses       190     190           

[khaeru]

Looks great.

In the future when we receive/review changes/additions to the tutorials, we can apply the same principle: in general, defining variables and then not later using them isn't desirable. It's especially bad in instructional material, because it interferes with the learner's process of forming a mental model of how the code works/should be used. For instance, if we say "Suppose A, B, and C. Combining B and C, we get…" —the reader is forced to keep part of their attention on "A was mentioned, I need to keep listening to understand what it's for."