Enable test_static_route for ipv6 only topo

[yanmo96]

Description of PR

Summary: Replace PR for https://github.com/sonic-net/sonic-mgmt/pull/21593, to get over the signature check

Type of change

• [x] Bug fix
• [ ] Testbed and Framework(new/improvement)
• [ ] New Test case
  • [ ] Skipped for non-supported platforms
• [x] Test case improvement

Back port request

• [ ] 202205
• [ ] 202305
• [ ] 202311
• [ ] 202405
• [x] 202411
• [x] 202505
• [x] 202511

Approach

What is the motivation for this PR?

To enable ipv6 only topo for test_static_route

How did you do it?

Have a check for ipv6 and use appropriate prefix for it.

How did you verify/test it?

Tested on ipv6 only testbed to verify

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation