Add support for new Okta TLD domains (okta-gov.com, okta.mil, okta-miltest.com, trex-govcloud.com)

[aniket-okta]

Summary

This PR adds support for new Okta TLD domains as part of the TG1 TLD Update initiative. The SDK now properly validates and accepts the following new domains:

• okta-gov.com - New production domain for OG1
• okta.mil - New production domain for OK15
• okta-miltest.com - New test domain for testing transition
• trex-govcloud.com - New government cloud domain

Changes Made

• Domain Validation: Updated Configuration.cs to include new domains in admin keyword validation
• Templates: Updated code generation templates to include new domains
• OpenAPI Specs: Updated management.yaml and management-noEnums.yaml with new domain enums

Testing

• Added negative test cases to ensure URLs with -admin are rejected for new domains
• Added positive test cases to ensure URLs without -admin are accepted for new domains
• All existing tests continue to pass

Security

Maintains existing security validations to prevent users from accidentally using admin URLs with the new domains.

Issue Reference

Addresses TG1 TLD Update requirements for okta-sdk-dotnet repository.

[prachi-okta]

LGTM