chore: suppress a Node.js deprecation warning (#16398)

Follow up nodejs/node#37302.

This PR suppress the following Node.js warning.

```console
% node -v
v18.10.0
% cd path/to/eslint
% node Makefile mocha
Running unit tests
(snip)

(node:63796) [DEP0147] DeprecationWarning: In future versions of Node.js,
fs.rmdir(path, { recursive: true }) will be removed. Use fs.rm(path, { recursive: true }) instead
(Use `node --trace-deprecation ...` to show where the warning was created)
```

And added compatibility condition to prevent the following error when using Node.js 14.13.1 or lower.

```console
% node -v
v14.13.1
% npx mocha tests/lib/eslint/flat-eslint.js
(snip)

1) FlatESLint
     lintFiles()
       multiple processors
         "after each" hook for "should lint only JavaScript blocks.":
     TypeError: fsp.rm is not a function
```

tests/lib/eslint/flat-eslint.js

@@ -2607,7 +2607,17 @@ describe("FlatESLint", () => {
             let id;
 
             beforeEach(() => (id = Date.now().toString()));
-            afterEach(async () => fsp.rmdir(root, { recursive: true, force: true }));
+
+            /*
+             * `fs.rmdir(path, { recursive: true })` is deprecated and will be removed.
+             * Use `fs.rm(path, { recursive: true })` instead.
+             * When supporting Node.js 14.14.0+, the compatibility condition can be removed for `fs.rmdir`.
+             */
+            if (typeof fsp.rm === "function") {
+                afterEach(async () => fsp.rm(root, { recursive: true, force: true }));
+            } else {
+                afterEach(async () => fsp.rmdir(root, { recursive: true, force: true }));
+            }
 
             it("should lint only JavaScript blocks.", async () => {
                 const teardown = createCustomTeardown({