[v1.15.x] Bump envoy-gloo, re-enabling support for REST EDS

[ben-taussig-solo]

Description

• Bump envoy-gloo to v1.26.5-patch2
• This bump contains updates to envoy-gloo's extensions_build_config.bzl which re-enable support for REST EDS, which was deprecated in upstream envoy v1.26
  • The envoy-gloo PR that made those changes can be found here: solo-io/envoy-gloo@206304f
• As discussed in Gloo fails after upgrade from 1.14.27 to 1.15.x #8968, users of 1.15.x (which depends on envoy-gloo v1.26.x) were previously unable to configure REST EDS
  • Please note that we currently do not recommend users to enable REST EDS, as it is significantly less performant and does not offer meaningful benefits as compared to the default GRPC XDS/EDS strategy

Testing steps

• I urge any reviewers to manually verify this behavior before approving this PR

Part 1: Validate that REST EDS is currently broken

1. Create a kind cluster:
   • kind create cluster --name kind
2. Install Gloo Edge 1.15.x with REST EDS enabled
   • helm install gloo gloo/gloo -n gloo-system --create-namespace --devel --version 1.15.18 --set settings.enableRestEds=true
3. Inspect the gateway-proxy deployment logs to confirm that errors are present
   • kubectl logs -n gloo-system deployment/gateway-proxy | grep "Didn't find a registered config subscription factory"
   • You should see many results

Part 2: Validate that the new changes resolve the issue

1. Build the envoy-gloo wrapper docker image and push it to the kind cluster, updating the gateway-proxy deployment to use the new image
   • VERSION=1.15.18-rest-eds-test make kind-reload-gloo-envoy-wrapper
2. Inspect the gateway-proxy deployment logs to confirm that errors are no longer present
   • kubectl logs -n gloo-system deployment/gateway-proxy | grep "Didn't find a registered config subscription factory"
   • You should see no results