The Flink extra currently combines a base pyarrow>=21.0.0 requirement with a
pyarrow<21.0.0 extra requirement. The attempted extra != 'flink' exclusion
does not prevent pip or uv from evaluating both requirements, so
feast[flink] is unsatisfiable.

This change puts the shared pyarrow>=16.1.0 lower bound in the base
dependencies and adds only pyarrow<21.0.0 to the Flink extra. A normal Feast
install can still select the latest PyArrow release, while a Flink install gets
the compatible >=16.1.0,<21.0.0 intersection.

The dependency regression check now lives in a focused test module that does
not import the Flink runtime.

Which issue(s) this PR fixes

Fixes #6583.

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s).
• My PR title follows conventional commits format.

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change

Validated with:

• the focused dependency test on Python 3.10;
• dry-run uv resolution for both .[flink] and the default project;
• a wheel build and inspection of its Requires-Dist metadata;
• Ruff check and format verification.

Release note

fix: Make the Flink extra installable by applying its PyArrow upper bound only when the extra is selected.