etcdserver: keep range request limit when sorting by key ascending

[jprieto-temporal]

The rangeLimit function was incorrectly setting limit=0 (fetch everything) for all sort operations, including the common case of sorting by key in ascending order. Since the backend already returns keys in lexicographically ascending order, we can preserve the limit for this case to improve performance and reduce memory usage.

This change modifies the condition to only fetch everything when:

• Sort order is not NONE AND
• Sort is not by key in ascending order (the default backend order) OR
• Revision filters are applied

Fixes #20745

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @jprieto-temporal. Thanks for your PR.

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