tests(conformance): enable HTTPRouteResponseHeaderModification

[programmer04]

What this PR does / why we need it:

Enable HTTPRouteResponseHeaderModification and do some small opportunistic improvements.

Report generated locally

apiVersion: gateway.networking.k8s.io/v1alpha1
date: "2024-05-20T22:08:30+02:00"
gatewayAPIChannel: experimental
gatewayAPIVersion: v1.1.0
implementation:
  contact:
  - github.com/kong/gateway-operator/issues/new/choose
  organization: Kong
  project: gateway-operator
  url: github.com/kong/gateway-operator
  version: v1.2.2-41-g293bfed
kind: ConformanceReport
mode: default
profiles:
- core:
    result: partial
    skippedTests:
    - GatewayInvalidTLSConfiguration
    - GatewayModifyListeners
    - GatewayWithAttachedRoutes
    - HTTPRouteHTTPSListener
    - HTTPRouteHeaderMatching
    - HTTPRouteHostnameIntersection
    - HTTPRouteInvalidBackendRefUnknownKind
    - HTTPRouteListenerHostnameMatching
    - HTTPRouteObservedGenerationBump
    statistics:
      Failed: 0
      Passed: 24
      Skipped: 9
  extended:
    result: success
    statistics:
      Failed: 0
      Passed: 1
      Skipped: 0
    supportedFeatures:
    - HTTPRouteResponseHeaderModification
    unsupportedFeatures:
    - GatewayHTTPListenerIsolation
    - GatewayPort8080
    - GatewayStaticAddresses
    - HTTPRouteBackendRequestHeaderModification
    - HTTPRouteBackendTimeout
    - HTTPRouteHostRewrite
    - HTTPRouteMethodMatching
    - HTTPRouteParentRefPort
    - HTTPRoutePathRedirect
    - HTTPRoutePathRewrite
    - HTTPRoutePortRedirect
    - HTTPRouteQueryParamMatching
    - HTTPRouteRequestMirror
    - HTTPRouteRequestMultipleMirrors
    - HTTPRouteRequestTimeout
    - HTTPRouteSchemeRedirect
  name: GATEWAY-HTTP
  summary: Core tests partially succeeded with 9 test skips. Extended tests succeeded.

Which issue this PR fixes

Closes #60