update least_time LB method availability for NGINX OSS

[yogarajalakshmi-s]

Proposed changes

NGINX Open Source v1.31.0 added least_time HTTP load balancing, previously Plus-only. This PR updates the docs to reflect that all least_time variants are now available in OSS: removes the Plus-only callout in upstream-settings.md, removes the Load balancing methods bullet from nginx-plus.md, and removes Plus-only labels from http-load-balancer.md. Stream least_time (TCP/UDP) remains Plus-only and is unchanged. This PR is related to nginx/nginx-gateway-fabric#5381

This PR fixes #2026

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[yogarajalakshmi-s]

opened it by mistake instead of changing the rebase in the existing PR #2031 . So closing this.