fix: ensure flatcar configs use transparent Azure CNI

[jackfrancis]

Reason for Change:

This PR ensures that new flatcar node pool-enabled clusters use transparent mode if using Azure CNI, as that's proven to be the only working Azure CNI implementation w/ flatcar.

No upgrade foo added here; we'll see if folks desire that, but for now, this is a cleaner change that doesn't try to be too tricky in "converting" existing clusters that were built w/ bridge mode prior to the systemd change that broke it.

Fixes #3920

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

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