Enable emptyDir volumes for Knative Services

[dprotaso]

Part of #11664

Proposed Changes

This switches the feature flag to enabled allowing users to
add emptyDir volumes to their containers.

This can be useful as a scratch space and for multiple
containers

Release Note

emptyDir volumes feature flag is now enabled by default

[dprotaso]

/assign @nader-ziada @psschwei

/cc @evankanderson

[codecov]

Codecov Report

Base: 86.47% // Head: 86.49% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (cf9c93f) compared to base (6264c1b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13405      +/-   ##
==========================================
+ Coverage   86.47%   86.49%   +0.02%     
==========================================
  Files         196      196              
  Lines       14551    14551              
==========================================
+ Hits        12583    12586       +3     
+ Misses       1669     1667       -2     
+ Partials      299      298       -1     

Impacted Files	Coverage Δ
pkg/apis/config/features.go	95.38% <100.00%> (ø)
pkg/reconciler/configuration/configuration.go	82.93% <0.00%> (-1.43%)	⬇️
pkg/apis/serving/k8s_validation.go	94.21% <0.00%> (+0.45%)	⬆️
pkg/autoscaler/statforwarder/leases.go	73.95% <0.00%> (+1.56%)	⬆️

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[psschwei]

/test upgrade-tests

[evankanderson]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, evankanderson

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