fix(nginx): Update proxy-buffer-size to 8k

This patch will resolve various issues with header sizes related to the
oauth2-proxy. That appear to result in 502s on nginx side due to large
headers.

infrastructure/nginx-system/release.yaml

@@ -87,6 +87,7 @@ data:
         hsts-include-subdomains: false
         enable-brotli: "true"
         use-http2: "true"
+        proxy-buffer-size: 8k
       replicaCount: 2
       minAvailable: 1
       resources: