next-lint: Use ESLint v9 by default (#71371)

### Why?

Since ESLint v9 was unblocked, we no longer need to pin installing v8 when running `next lint` for the first time.

Author: devjiwonchoi

packages/next/src/lib/eslint/runLintCheck.ts

@@ -432,8 +432,8 @@ export async function runLintCheck(
           if (deps.missing.length > 0) {
             deps.missing.forEach((dep) => {
               if (dep.pkg === 'eslint') {
-                // eslint v9 has breaking changes, so lock to 8 until dependency plugins fully support v9.
-                dep.pkg = 'eslint@^8'
+                // pin to v9 to avoid breaking changes
+                dep.pkg = 'eslint@^9'
               }
             })