[action] [PR:21393] Disable default route in dpu

[mssonicbld]

Description of PR

Summary:
Fixes #21392

Type of change

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

Back port request

• 202205
• 202305
• 202311
• 202405
• 202411
• 202505

Approach

What is the motivation for this PR?

minigraph for smartswitch topo should not have gateway in DHCP_IPV4_SERVER table. f gateway is not present, "router" dhcp option won't be included in the response and client won't install default route. Please see PR sonic-net/sonic-buildimage#21462 for the design.

How did you do it?

Remove gateway option from generated golden config

How did you verify/test it?

Run "./testbed-cli.sh deploy-mg vms-kvm-t1-smartswitch-ha ./lab ./password.txt"
Verify "gateway" option is not in DHCP_IPV4_SERVER
Verified no default route over midplane in DPU by show ip route.

`admin@dpu00:~$ show ip route
Codes: K - kernel route, C - connected, S - static, R - RIP,
O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
T - Table, v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
F - PBR, f - OpenFabric,

• selected route, LICENSE README.md SECURITY.md ansible azure-pipelines.yml docs pylintrc pyproject.toml sdn_tests setup-container.sh sonic_dictionary.txt spytest test_reporting tests - FIB route, q - queued route, r - rejected route

S>*0.0.0.0/0 [1/0] via 20.0.200.14, Ethernet0, 17:04:20
C>*20.0.200.0/28 is directly connected, Ethernet0, 17:04:20
K 20.0.200.0/28 [0/0] is directly connected, Ethernet0, 17:04:23
C>*169.254.200.0/24 [0/1024] is directly connected, eth0-midplane, 17:04:27
K 240.127.1.0/24 [0/0] is directly connected, docker0, inactive 17:04:27
admin@dpu00:~$ Connection to 169.254.200.1 closed by remote host.
`

Any platform specific information?

Specific to SmartSwitch

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

Documentation

[mssonicbld]

Original PR: #21393

[mssonicbld]

/azp run

[azure-pipelines]

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