Add WEIGHTED_MAGLEV to localityLbPolicy enum, and update associated docstrings.

This PR adds WEIGHTED_MAGLEV to the enum for localist_lb_policy, resolving b/272540261 and unblocking NetLB Weighted Load Balancing GA.

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compute: added `WEIGHED_MAGLEV` to `locality_lb_policy` enum for backend service resources.

GoogleCloudPlatform / terraform-validator