Update deprecation horizon to 8.1

[mamhoff]

We've just introduced some new deprecation warnings that should take effect after we release Alchemy 8.0, not when it is released.

We can also use 9.0, just comment.

I've looked for deleteable deprecated code and couldn't find any.

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.13%. Comparing base (7711d16) to head (bb30b43).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3427   +/-   ##
=======================================
  Coverage   97.13%   97.13%           
=======================================
  Files         279      279           
  Lines        7369     7374    +5     
=======================================
+ Hits         7158     7163    +5     
  Misses        211      211           

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[tvdeyen]

9.0