Add "Pending" reason and document usage for condition "Accepted", status: "Unknown" on all resources

[mikemorris]

What would you like to be added:

• Add a new "Pending" status condition reason to match the existing ListenerConditionReason
  • GatewayClassConditionReason
  • GatewayConditionReason
  • RouteConditionReason
• Document that ListenerReasonPending should be used when the ListenerConditionAccepted status is Unknown.
• Deprecate GatewayClassReasonWaiting and standardize description
• Deprecate GatewayReasonNotReconciled and standardize description

Why this is needed:

GEP-1364 clarified that the Accepted condition should always be set, even when the status would be "Unknown", and the reason field on Condition is required.