fix: packages/@vuepress/core/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-SSRI-1085630

packages/@vuepress/core/package.json

@@ -35,7 +35,7 @@
     "cache-loader": "^3.0.0",
     "chokidar": "^2.0.3",
     "connect-history-api-fallback": "^1.5.0",
-    "copy-webpack-plugin": "^5.0.2",
+    "copy-webpack-plugin": "^6.0.0",
     "core-js": "^3.6.4",
     "cross-spawn": "^6.0.5",
     "css-loader": "^2.1.1",
@@ -56,7 +56,7 @@
     "vue-template-compiler": "^2.6.10",
     "vuepress-html-webpack-plugin": "^3.2.0",
     "vuepress-plugin-container": "^2.0.2",
-    "webpack": "^4.8.1",
+    "webpack": "^5.0.0",
     "webpack-chain": "^6.0.0",
     "webpack-dev-server": "^3.5.1",
     "webpack-merge": "^4.1.2",