Update step names for consistency

Rename 'Cache PD/TiKV Docker image' to 'Pull and cache PD/TiKV Docker image'
to match the TiDB step naming and clarify that these steps both pull
and cache the images.

Author: zph

.github/workflows/main.yml

@@ -253,7 +253,7 @@ jobs:
           cache-from: type=gha,scope=tidb-v${{ steps.extract-tidb-version.outputs.version }}
           cache-to: type=gha,mode=max,scope=tidb-v${{ steps.extract-tidb-version.outputs.version }}
 
-      - name: Cache PD Docker image using Buildx
+      - name: Pull and cache PD Docker image using Buildx
         if: contains(matrix.target, 'tidb')
         uses: docker/build-push-action@v5
         continue-on-error: false
@@ -268,7 +268,7 @@ jobs:
           cache-from: type=gha,scope=pd-v${{ steps.extract-tidb-version.outputs.version }}
           cache-to: type=gha,mode=max,scope=pd-v${{ steps.extract-tidb-version.outputs.version }}
 
-      - name: Cache TiKV Docker image using Buildx
+      - name: Pull and cache TiKV Docker image using Buildx
         if: contains(matrix.target, 'tidb')
         uses: docker/build-push-action@v5
         continue-on-error: false