Tech debt: Use AutoFlEx for aws_rds_export_task

[ewbankkit]

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the library.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Description

The aws_rds_export_task resource was one of the first to use terraform-plugin-framework, before the introduction of AutoFlEx.
This PR refactors to use AutoFlEx.

Relations

Relates #38907.
Relates #42741.

Output from Acceptance Testing

% make testacc TESTARGS='-run=TestAccRDSExportTask_' PKG=rds
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.24.3 test ./internal/service/rds/... -v -count 1 -parallel 20  -run=TestAccRDSExportTask_ -timeout 360m -vet=off
2025/05/28 14:58:59 Creating Terraform AWS Provider (SDKv2-style)...
2025/05/28 14:58:59 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccRDSExportTask_basic
=== PAUSE TestAccRDSExportTask_basic
=== RUN   TestAccRDSExportTask_optional
=== PAUSE TestAccRDSExportTask_optional
=== CONT  TestAccRDSExportTask_basic
=== CONT  TestAccRDSExportTask_optional
--- PASS: TestAccRDSExportTask_basic (1345.80s)
--- PASS: TestAccRDSExportTask_optional (1771.69s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/rds	1776.639s

[github-actions]

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[johnsonaj]

LGTM 🚀

[github-actions]

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