r/aws_redshift_cluster: Fix master_username validation

[jar-b]

Description

Fixes validation on the master_username argument by allowing additional special characters permitted by the AWS API but incorrectly flagged by plan time validation.

Relations

Closes #41437
Ported from #41522

Output from Acceptance Testing

% make testacc PKG=redshift TESTS=TestAccRedshiftCluster_masterUsername
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.23.5 test ./internal/service/redshift/... -v -count 1 -parallel 20 -run='TestAccRedshiftCluster_masterUsername'  -timeout 360m -vet=off
2025/02/24 15:28:42 Initializing Terraform AWS Provider...

--- PASS: TestAccRedshiftCluster_masterUsername_invalid (2.85s)
--- PASS: TestAccRedshiftCluster_masterUsername (361.88s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/redshift   368.539s

[github-actions]

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