chore(deps): bump next to 15.4.8 in root and test dirs (#14811)

Follow up to #14807.

The monorepo root and test directories specified older versions of
`next`. Bumps to `15.4.8` accordingly.

The `@payloadcms/plugin-multi-tenant` package also unnecessarily
specifies a `next` version in its peer dependencies. This has been
removed.

Author: jacobsfletch

package.json

@@ -177,7 +177,7 @@
     "minimist": "1.2.8",
     "mongodb-memory-server": "10.1.4",
     "mongoose": "8.15.1",
-    "next": "15.4.7",
+    "next": "15.4.8",
     "open": "^10.1.0",
     "p-limit": "^5.0.0",
     "pg": "8.16.3",

packages/next/package.json

@@ -141,7 +141,7 @@
   },
   "peerDependencies": {
     "graphql": "^16.8.1",
-    "next": "^15.2.3",
+    "next": "^15.4.8",
     "payload": "workspace:*"
   },
   "engines": {

packages/plugin-multi-tenant/package.json

@@ -98,7 +98,6 @@
   },
   "peerDependencies": {
     "@payloadcms/ui": "workspace:*",
-    "next": "^15.2.3",
     "payload": "workspace:*"
   },
   "publishConfig": {