fix(net-schema): no warn when skipping schema check on non-netplan

[blackboxsw]

Proposed Commit Message

    fix(net-schema): no warn when skipping schema check on non-netplan
    
    Avoid warning when network-config schema is version 2 and we
    are on non-netplan systems because cloud-init doesn't yet
    have static JSON network-config schema defined yet for
    version 2.
    
    Also fixes integration test to expect python3 netplan API
    on Ubuntu Mantic and newer.
    
    Fixes GH-4814

Additional Context

Also failed our Jenkins test runners
https://jenkins.canonical.com/server-team/view/cloud-init/job/cloud-init-integration-mantic-lxd_container/139/testReport/junit/tests.integration_tests.cmd.test_schema/TestSchemaDeprecations/test_network_config_schema_validation/

Test Steps

Validation performed with this PR on a system with no netplan python imports available
Notice:

1. happy cloud-init status without warnings
2. skipping netplan schema validation debug log
3. schema validation reporting same reason for skipped validation

root@nnn:~# grep netplan /var/log/cloud-init.log 
2024-01-29 22:09:53,263 - schema.py[DEBUG]: Skipping netplan schema validation. No netplan available
2024-01-29 22:09:53,265 - distros[DEBUG]: Selected renderer 'netplan' from priority list: ['netplan', 'eni', 'sysconfig']
2024-01-29 22:09:53,265 - network_state.py[DEBUG]: Passthrough netplan v2 config
2024-01-29 22:09:53,265 - netplan.py[DEBUG]: V2 to V2 passthrough
2024-01-29 22:09:53,265 - util.py[DEBUG]: Writing to /etc/netplan/50-cloud-init.yaml - wb: [600] 389 bytes
2024-01-29 22:09:53,265 - netplan.py[DEBUG]: skipping call to `netplan generate`. reason: identical netplan config
root@nnn:~# cloud-init schema --system
Found cloud-config data types: user-data, network-config

1. user-data at /var/lib/cloud/instances/77a8ebd1-cae8-4727-83ff-330bf7c9c5e4/cloud-config.txt:
  Valid schema user-data

2. network-config at /var/lib/cloud/instances/77a8ebd1-cae8-4727-83ff-330bf7c9c5e4/network-config.json:
Skipping network-config schema validation. No network schema for version: 2

Checklist

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[blackboxsw]

CC: @dermotbradley thanks for the issue 👍

[blackboxsw]

#4814

[TheRealFalcon]

LGTM!