Switch to ESLint v9 and flat config (#3558)

Since PR #3551 was not yet complete, I made my own attempt.

1. Update to ESLint v9.
2. Replace deprecated `.eslintrc.json` and `.eslintignore` by flat
config `eslint.config.mjs`.
3. Adapt `check_config.js` to use flat config.
4. Since `eslint-plugin-import` still doesn't support ESLint v9 I
removed it. We can add it back when it does support v9.
5. Run tests `npm run check:js` and `npm run config:check`.
6. In order not to overload this PR, I have not yet activated more
additional rules - there are some useful ones in the new plugin
`@eslint/js`.

@bugsounet, please don't take it as an offence that I have created a
competing PR. The migration to ESLint v9 has been burning under my nails
for some time.

package.json

@@ -59,7 +59,7 @@
 		"ansis": "^3.3.2",
 		"console-stamp": "^3.1.2",
 		"envsub": "^4.1.0",
-		"eslint": "^8.57.0",
+		"eslint": "^9.11.1",
 		"express": "^4.21.0",
 		"express-ipfilter": "^1.3.2",
 		"feedme": "^2.0.2",
@@ -75,13 +75,12 @@
 		"systeminformation": "^5.23.5"
 	},
 	"devDependencies": {
+		"@eslint/js": "^9.11.1",
 		"@stylistic/eslint-plugin": "^2.8.0",
 		"cspell": "^8.14.4",
-		"eslint-plugin-import": "^2.30.0",
 		"eslint-plugin-jest": "^28.8.3",
 		"eslint-plugin-jsdoc": "^50.2.4",
 		"eslint-plugin-package-json": "^0.15.3",
-		"eslint-plugin-unicorn": "^55.0.0",
 		"express-basic-auth": "^1.2.1",
 		"husky": "^9.1.6",
 		"jest": "^29.7.0",