Enable expressions router by default (in test manifests) & remove the feature gate

[programmer04]

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Problem Statement

Enable expression router by default, it obsoletes

• Align router defaults with Kong upstream #3693

The expression-based routers, introduced in Kong Gateway 3.0, have higher performance and allow more precise priority specification than the traditional router. By using the expression-based router, Kong Ingress Controller can reduce most of the generated routes with regular expression matches on path, improving the router's performance.

For KIC 3.0 targeted Kong Gateway version is >=3.4.1

Proposed Solution

• expression router is enabled by default

Additional information

Related issues that should be resolved

• Support TLSRoute with expression router #4540
• Support L4 proxy CRDs in ExpressionRoutes mode #4312
• Expression Router: Validate Predicates when creating predicates #3822
• Gateway API conformance HTTPRouteHeaderMatching case #3731

Acceptance Criteria

• expressions router is fully functional
• expression router is enabled by default