Stabilize `generic-not-last-base-class` (`PYI059`)
Summary
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The tests were already in the right place, I just updated the documentation
slightly to reflect the discussion
[here](https://github.com/astral-sh/ruff/pull/18519#issuecomment-2956344254) and
on Discord.
Documentation: https://docs.astral.sh/ruff/rules/generic-not-last-base-class
Tests: https://github.com/astral-sh/ruff/blob/brent/release-0.12.0/crates/ruff_linter/src/rules/flake8_pyi/mod.rs#L51-L52
Test Plan
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Existing tests
