docs: Update v20 upgrade guide to clarify that `"CONFIG_MAP"` is not a supported access type #1858

Job	Run time
Collect workflow inputs	4s
Min TF pre-commit (.)	23s
Max TF pre-commit	1m 28s
Min TF pre-commit (./modules/eks-managed-node-group)	26s
Min TF pre-commit (./modules/aws-auth)	11s
Min TF pre-commit (./modules/karpenter)	20s
Min TF pre-commit (./modules/fargate-profile)	23s
Min TF pre-commit (./modules/self-managed-node-group)	21s
Min TF pre-commit (./modules/hybrid-node-role)	20s
Min TF pre-commit (./tests/eks-hybrid-nodes)	25s
Min TF pre-commit (./tests/eks-managed-node-group)	30s
Min TF pre-commit (./tests/fast-addons)	23s
Min TF pre-commit (./tests/self-managed-node-group)	25s
Min TF pre-commit (./tests/eks-fargate-profile)	22s
Min TF pre-commit (./tests/user-data)	12s
Min TF pre-commit (./examples/eks-hybrid-nodes)	28s
Min TF pre-commit (./examples/eks-managed-node-group)	26s
Min TF pre-commit (./examples/eks-auto-mode)	24s
Min TF pre-commit (./examples/karpenter)	26s
Min TF pre-commit (./examples/self-managed-node-group)	23s
	8m 20s

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