feat: Add node group configuration capability

[waelghaith]

Description

• Add node_group_configuration support for cluster‑mode replication groups and wire it through the wrapper.
• Bump AWS provider minimum to >= 6.20.0 across root, modules, wrappers, and examples.
• Update terraform-docs outputs.

Motivation and Context

• The AWS provider added node_group_configuration (>= 6.20.0) for cluster‑mode replication groups, needed for shard AZ pinning and slot ranges. The module didn’t expose this capability.
• This support is required when restoring replication groups from snapshot_arns, since AWS requires node_group_configuration (including slots) for that workflow.

Breaking Changes

• AWS provider minimum is now >= 6.20.0.
• preferred_cache_cluster_azs default changed from [] to null (argument omitted by default).

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

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