Graduate FleetAllocationOverflow to Stable

[Kalaiselvi84]

Is your feature request related to a problem? Please describe.
The "FleetAllocationOverflow" feature has been in Beta since 1.37.0.

Describe the solution you'd like
Move it to stable, since it's been in Beta and turned on by default for a while.

Describe alternatives you've considered
Remove the functionality.

Additional context
Task list:

agones/pkg/util/runtime/features.go

Lines 93 to 98 in 065052a

	// To promote a feature from beta->GA:
	// * remove all places consuming the feature gate and fold logic to true
	// * consider cleanup - often folding a gate to true allows refactoring
	// * invert the "new alpha feature" steps above
	//
	// In each of these, keep the feature sorted by descending maturity then alphabetical