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

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-CROSSSPAWN-8303230

packages/@vuepress/core/package.json

@@ -37,7 +37,7 @@
     "connect-history-api-fallback": "^1.5.0",
     "copy-webpack-plugin": "^5.0.2",
     "core-js": "^3.6.4",
-    "cross-spawn": "^6.0.5",
+    "cross-spawn": "^7.0.5",
     "css-loader": "^2.1.1",
     "file-loader": "^3.0.1",
     "js-yaml": "^3.13.1",
@@ -58,7 +58,7 @@
     "vuepress-plugin-container": "^2.0.2",
     "webpack": "^4.8.1",
     "webpack-chain": "^6.0.0",
-    "webpack-dev-server": "^3.5.1",
+    "webpack-dev-server": "^4.6.0",
     "webpack-merge": "^4.1.2",
     "webpackbar": "3.2.0"
   },