Update dual-stack support for DigitalOcean with DO CCM

[rastislavs]

After DigitalOcean CCM support is added (#10837), review dual-stack support for DigitalOcean with DO CCM:

• Investigate dual-stack capabilities of CCM (e.g. turn on necessary flags if they exist)
• Ensure dual-stack works fine with CCM enabled
• Update provider’s dual-stack documentation

[sachintiptur]

IPv6 is completely not supported in DO CCM yet. Raised an issue #555.

[sachintiptur]

PR #561 raised in DO CCM upstream.