resource/aws_docdb_cluster: Allow adding/modifying serverless_v2_scaling_configuration without replacement

[tobias-schuemann]

Description

Closes #44911

This PR fixes a bug where adding or modifying serverless_v2_scaling_configuration on an existing
DocumentDB cluster forced a destructive cluster replacement instead of performing an in-place update.

Root Cause

The resource had a CustomizeDiff function that forced replacement (ForceNew) whenever the serverless
configuration block was added or removed. However, the AWS ModifyDBCluster API supports in-place updates
for this configuration.

Solution

• Removed the CustomizeDiff block that was forcing replacement
• Added DiffSuppressFunc to handle AWS's behavior where the configuration persists even when removed
  from Terraform code
• Follows the established pattern from aws_rds_cluster for the same serverless configuration

Changes

• cluster.go: Removed CustomizeDiff, added DiffSuppressFunc at block and attribute levels
• cluster_test.go: Added comprehensive tests for add/modify/remove scenarios
• Documentation: Updated to reflect persistence behavior
• Changelog: Added bug fix entry

User Impact

After this change, users can:

• Add serverless_v2_scaling_configuration to existing clusters (in-place update)
• Modify min_capacity and max_capacity values (in-place update)
• Remove the configuration block from code without perpetual drift warnings
• The configuration remains active in AWS (as documented)

Output from Acceptance Testing

make testacc TESTS=TestAccDocDBCluster_serverless PKG=docdb
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
make: Running acceptance tests on branch: 🌿 main 🌿...
TF_ACC=1 go1.24.10 test ./internal/service/docdb/... -v -count 1 -parallel 20 -run='TestAccDocDBCluster_serverless' -timeout 360m -vet=off
2025/11/13 00:11:37 Creating Terraform AWS Provider (SDKv2-style)...
2025/11/13 00:11:37 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN TestAccDocDBCluster_serverless
=== PAUSE TestAccDocDBCluster_serverless
=== RUN TestAccDocDBCluster_serverlessAddToExisting
=== PAUSE TestAccDocDBCluster_serverlessAddToExisting
=== CONT TestAccDocDBCluster_serverless
=== CONT TestAccDocDBCluster_serverlessAddToExisting
--- PASS: TestAccDocDBCluster_serverlessAddToExisting (135.21s)
--- PASS: TestAccDocDBCluster_serverless (155.34s)
PASS
ok github.com/hashicorp/terraform-provider-aws/internal/service/docdb 160.331s

[github-actions]

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[github-actions]

✅ Thank you for correcting the previously detected issues! The maintainers appreciate your efforts to make the review process as smooth as possible.