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.

CHANGELOG.md

@@ -23,6 +23,7 @@ _This release is scheduled to be released on 2024-10-01._
 
 - [core] removed installer only files (#3492)
 - [core] removed raspberry object from systeminformation (#3505)
+- [linter] removed `eslint-plugin-import`, because it doesn't support ESLint v9. We will reenter it later when it does.
 
 ### Updated
 
@@ -32,6 +33,7 @@ _This release is scheduled to be released on 2024-10-01._
 - [core] Allow custom modules positions by scanning index.html for the defined regions, instead of hard coded (PR #3518 fixes issue #3504)
 - [core] Detail optimizations in `config_check.js`
 - [core] Updated minimal needed node version in `package.json` (currently v20.9.0)
+- [linter] Switch to ESLint v9 and flat config and replace `eslint-plugin-unicorn` by `@eslint/js`
 
 ### Fixed