avoid string slice panic in version check

[brenthc]

See internal task referencing this PR.

Closes #22542.
Closes #20860.

Unit tests

% go test -v ./internal/service/rds '-run=TestCompareActualEngineVersion'
=== RUN   TestCompareActualEngineVersion
=== PAUSE TestCompareActualEngineVersion
=== CONT  TestCompareActualEngineVersion
=== RUN   TestCompareActualEngineVersion/point_version_upgrade
=== RUN   TestCompareActualEngineVersion/minor_version_upgrade
=== RUN   TestCompareActualEngineVersion/major_version_upgrade
=== RUN   TestCompareActualEngineVersion/aurora_upgrade
=== RUN   TestCompareActualEngineVersion/aurora_upgrade_-_used_to_crash
=== RUN   TestCompareActualEngineVersion/oracle
=== RUN   TestCompareActualEngineVersion/oracle_-_used_to_crash
--- PASS: TestCompareActualEngineVersion (0.00s)
    --- PASS: TestCompareActualEngineVersion/point_version_upgrade (0.00s)
    --- PASS: TestCompareActualEngineVersion/minor_version_upgrade (0.00s)
    --- PASS: TestCompareActualEngineVersion/major_version_upgrade (0.00s)
    --- PASS: TestCompareActualEngineVersion/aurora_upgrade (0.00s)
    --- PASS: TestCompareActualEngineVersion/aurora_upgrade_-_used_to_crash (0.00s)
    --- PASS: TestCompareActualEngineVersion/oracle (0.00s)
    --- PASS: TestCompareActualEngineVersion/oracle_-_used_to_crash (0.00s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/rds	3.998s

Acceptance tests

% make testacc TESTARGS='-run=TestAccRDSCluster_allowMajorVersionUpgrade\|TestAccRDSCluster_basic' PKG=rds          
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/rds/... -v -count 1 -parallel 20  -run=TestAccRDSCluster_allowMajorVersionUpgrade\|TestAccRDSCluster_basic -timeout 180m
=== RUN   TestAccRDSCluster_basic
=== PAUSE TestAccRDSCluster_basic
=== RUN   TestAccRDSCluster_allowMajorVersionUpgrade
=== PAUSE TestAccRDSCluster_allowMajorVersionUpgrade
=== RUN   TestAccRDSCluster_allowMajorVersionUpgradeWithCustomParametersApplyImm
=== PAUSE TestAccRDSCluster_allowMajorVersionUpgradeWithCustomParametersApplyImm
=== RUN   TestAccRDSCluster_allowMajorVersionUpgradeWithCustomParameters
=== PAUSE TestAccRDSCluster_allowMajorVersionUpgradeWithCustomParameters
=== CONT  TestAccRDSCluster_basic
=== CONT  TestAccRDSCluster_allowMajorVersionUpgradeWithCustomParametersApplyImm
=== CONT  TestAccRDSCluster_allowMajorVersionUpgradeWithCustomParameters
=== CONT  TestAccRDSCluster_allowMajorVersionUpgrade
--- PASS: TestAccRDSCluster_basic (128.37s)
--- PASS: TestAccRDSCluster_allowMajorVersionUpgrade (1318.20s)
--- PASS: TestAccRDSCluster_allowMajorVersionUpgradeWithCustomParametersApplyImm (1896.45s)
--- PASS: TestAccRDSCluster_allowMajorVersionUpgradeWithCustomParameters (1985.38s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/rds	1988.948s

[ewbankkit]

Needs backporting to release/3.x.

[ewbankkit]

LGTM 🚀.

[ewbankkit]

@brenthc Thanks for the contribution 🎉 👏.

[github-actions]

This functionality has been released in v4.1.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.