Update pip devcontainers to UCX 1.21

Author: bdice

.devcontainer/cuda12.9-pip/devcontainer.json

@@ -5,7 +5,7 @@
     "args": {
       "CUDA": "12.9",
       "PYTHON_PACKAGE_MANAGER": "pip",
-      "BASE": "rapidsai/devcontainers:26.12-cpp-cuda12.9-ucx1.19.0-openmpi5.0.10"
+      "BASE": "rapidsai/devcontainers:26.12-cpp-cuda12.9-ucx1.21.0-openmpi5.0.10"
     },
     "cacheFrom": [
       "ghcr.io/rapidsai/cuvs/devcontainer:26.12-cuda12.9-pip"

.devcontainer/cuda13.3-pip/devcontainer.json

@@ -5,7 +5,7 @@
     "args": {
       "CUDA": "13.3",
       "PYTHON_PACKAGE_MANAGER": "pip",
-      "BASE": "rapidsai/devcontainers:26.12-cpp-cuda13.3-ucx1.19.0-openmpi5.0.10"
+      "BASE": "rapidsai/devcontainers:26.12-cpp-cuda13.3-ucx1.21.0-openmpi5.0.10"
     },
     "cacheFrom": [
       "ghcr.io/rapidsai/cuvs/devcontainer:26.12-cuda13.3-pip"