Network: Rework IPv6 check.

[maxgio92]

The previous check was requiring the test shell builtin to check interface with IPv6 under procfs.
The same has been achieved with this commit using go "os" standard library.
Furthermore a preliminary check on the kernel-level setting to disable IPv6 is introduced, such that id IPv6 is disabled as kernel feature no further checks on interfaces is done in order to check if IPv6 is enabled.

Fixes #12903

What this PR does / why we need it:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• CVE Report (Scanner found CVE and adding report)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

fixes #12903

How Has This Been Tested?

This net test has been enabled under a specific build flag test_ipv6.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added unit and/or e2e tests to cover my changes.
• All new and existing tests passed.

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[Gacko]

/triage accepted
/kind feature
/priority backlog

Basically looks good to me, ping me as soon as you're ready for review.

[maxgio92]

Thank you @Gacko!

I've just enabled the test that was commented for IPV6, to be run executed only with the Go build tag test_ipv6 . WDYT?

[Gacko]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 89luca89, Gacko, maxgio92, prometherion

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