deps: bump @actions/core from 1.2.7 to 1.6.0 (#406)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

package-lock.json

@@ -40,7 +40,7 @@
   },
   "homepage": "https://github.com/peaceiris/actions-mdbook#readme",
   "dependencies": {
-    "@actions/core": "^1.2.7",
+    "@actions/core": "^1.6.0",
     "@actions/exec": "^1.0.4",
     "@actions/io": "^1.1.0",
     "@actions/tool-cache": "^1.6.1",