FIX Elkan k-means does not stop if tol=0

[kno10]

K-means convergence is still different between "full" k-means and "elkan" k-means.

The fix in #15831 is incomplete. Compare:

scikit-learn/sklearn/cluster/_kmeans.py

Lines 441 to 442 in 4fe4d27

	center_shift_total = squared_norm(centers_old - centers)
	if center_shift_total <= tol:

and

scikit-learn/sklearn/cluster/_k_means_elkan.pyx

Line 233 in 4fe4d27

center_shift = np.sqrt(np.sum((centers_ - new_centers) ** 2, axis=1))

scikit-learn/sklearn/cluster/_k_means_elkan.pyx

Lines 249 to 250 in 4fe4d27

	center_shift_total = np.sum(center_shift ** 2)
	if center_shift_total < tol:

it should be noted that in the second version, it would likely make sense to first use squared_norm, and then separate the square root, rather than taking the square of the rooted values below.
But in this PR I'm just pointing to a single character. One tests <= and the other tests <.
With tol=0 this means that "full" may stop when the clusters stop moving, while with elkan it never stops then, but always takes all iterations.

I do not think this is the best stopping criterion. If a numerical issue arises in computing the center shifts, this may cause the algorithm to always take the maximum number of iterations. The classic termination criterion for k-means is different: stop if no object is relabeled. That is more reliable.

[jeremiedbb]

lgtm. Please add a what's new entry.

[kno10]

Extended a unit test that failed before my change (300 vs. 7 iterations; simply by adding tol=0 to the existing test case), and a changelog entry.

[rth]

Could you please add an empty commit to re-trigger CI (git commit --allow-empty)?
The existing failing build fails to load so I don't know if it could be related. Otherwise LGTM.

[kno10]

Re-triggered CI, checks passed.

[rth]

Thanks @kno10 !