Update memberlist and dskit

[pracucci]

What this PR does

Update memberlist and dskit to grafana/dskit#798 and grafana/memberlist#11.

Which issue(s) this PR fixes or relates to

N/A

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.

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Note

Updates dskit and memberlist to add configurable multi-node push/pull; bridges now push/pull with 2 nodes per interval (others with 1).

• Memberlist (vendor fork):
  • Add Config.PushPullNodes with defaults set to 1 in DefaultLANConfig, DefaultWANConfig, and DefaultLocalConfig.
  • Update pushPull() to select N random live nodes and sync with each based on PushPullNodes.
• dskit (vendor):
  • In vendor/github.com/grafana/dskit/kv/memberlist/memberlist_client.go, set mlCfg.PushPullNodes to 2 for NodeRoleBridge, otherwise 1.
• Dependencies:
  • Bump github.com/grafana/dskit and replace github.com/hashicorp/memberlist with newer github.com/grafana/memberlist fork revisions.

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