Update required pnpm manager version

I update pnpm because the required version is quite old and
some devs are using newer version anyways and mixing versions
causes strange changelogs in package-lock.yml

I ran corepack up but I removed the sha from the version
because the previous version didn't have it as well.
[MAILPOET-6191]

package.json

@@ -14,7 +14,7 @@
     "prettier": "2.6.2",
     "typescript": "^5.0.2"
   },
-  "packageManager": "pnpm@8.5.1",
+  "packageManager": "pnpm@8.15.9",
   "volta": {
     "node": "19.7.0"
   },