remove force natgateway featuregate

[kon-angelo]

How to categorize this PR?

/area control-plane
/kind technical-debt
/platform azure

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

Remove ForceNatGateway feature gate after the updated announcement by Microsoft

[github-actions]

This change reverts the feature that automatically enabled Azure NAT Gateway for new shoot clusters. The ForceNatGateway feature gate, along with the associated admission controller logic and tests, has been removed. This change gives users explicit control over their outbound connectivity configuration by no longer defaulting to enabling a NAT Gateway. Documentation has also been updated to reflect this removal, ensuring guidance is accurate.

Walkthrough

• Revert: The functionality that automatically enabled Azure NAT Gateway for new shoot clusters has been removed. Users creating new clusters will no longer have a NAT Gateway configured by default and must explicitly define their outbound connectivity solution if needed.
• Documentation: User guides have been updated to remove mentions of the automatic NAT Gateway feature, ensuring documentation accurately reflects the current cluster creation behavior.

_{Model: gemini-2.5-pro | Prompt Tokens: 6020 | Completion Tokens: 158}

[hebelsan]

/lgtm