refactor!: Rename primary_keys to primary_key

[borchero]

Motivation

In a recent discussion, @MariusMerkleQC rightly pointed out that a data frame has a "primary key", regardless of how many columns the primary key spans. Using the plural might lead to some confusion.

Changes

• Rename primary_keys to primary_key everywhere

Let's wait for this PR until we release v2.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (20c89f4) to head (4a85b0b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #85   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           50        50           
  Lines         2901      2901           
=========================================
  Hits          2901      2901           

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