Move `FleetAutoscaleRequestMetaData` to Beta

[markmandel]

Is your feature request related to a problem? Please describe.
The FleetAutoscaleRequestMetaData feature gate has been in Alpha since Agones v1.48.0. It enables the Webhook Autoscaler to send fleet metadata (name, labels, annotations) with the autoscaling request, which is essential for more complex scaling logic.

The feature has been available for several releases and appears stable. E2E testing coverage was tracked in #4117.

Describe the solution you'd like
I would like to promote FleetAutoscaleRequestMetaData to Beta.

Following the feature stage progression steps defined in pkg/util/runtime/features.go, the following changes are required:

• Update pkg/util/runtime/features.go: Change the default value of FleetAutoscaleRequestMetaData to true.
• Update documentation: Move the feature from the "Alpha" table to the "Beta" table in the Feature Stages guide, and update any other documentation references.
• Update Helm charts (if applicable) to reflect the new default state (though typically handled by the code default).
• Ensure end-to-end tests are passing and cover the feature usage.

Describe alternatives you've considered
Leaving the feature in Alpha. However, promoting it to Beta (enabled by default) will encourage wider usage and validation, and reflects its maturity.

Additional context

• Feature Gate: FleetAutoscaleRequestMetaData
• Introduced in: v1.48.0
• Relevant Issues: Extend Webhook autoscaler to send fleet metadata with the request #3951 (Feature Request), end to end test: Webhook autoscaler to with fleet metadata #4117 (E2E Tests)

[sugar-cat7]

@markmandel
Hi! I'll take this on!