Updates in the calico config and template to support disabling ipv4 pools during pure ipv6 deployment

[AbhiiTen]

What type of PR is this?
/kind bug
/kind feature

What this PR does / why we need it:
Currently, when attempting a pure IPv6 deployment, Calico still attempts to allocate IPv4 addresses and utilize IPv4 CIDR pools
This PR updates the Calico configurations to correctly handle pure IPv6 stacks by:

1. Setting CALICO_IPV4POOL_CIDR to "none" in the node environment variables when IPv4 is disabled.
2. Updating the IP assignment logic in calico-config.yml.j2 to explicitly set assign_ipv4 to "false" and assign_ipv6 to "true" during pure IPv6 deployments.

Which issue(s) this PR fixes:
Fixes #13157

Special notes for your reviewer:
Please double-check the Jinja2 logic for dual-stack vs single-stack assignments to ensure it doesn't break existing IPv4-only or Dual-Stack clusters.
Does this PR introduce a user-facing change?:

- Calico network plugin now correctly supports disabling IPv4 pools during pure IPv6 deployments.
   - Setting CALICO_IPV4POOL_CIDR to "none" in the node environment variables when IPv4 is disabled.
   - Updating the IP assignment logic in calico-config.yml.j2 to explicitly set assign_ipv4 to "false" and assign_ipv6 to "true" during pure IPv6 deployments.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: AbhiiTen / name: AbhiiTen (21646cd, 4dd5014)
• ✅ login: AbhiiTen / name: root (51534ef)

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/lgtm