Decorate this as classic in README and recommend EX

[mxandreas]

This is only updated to the README to reflect that we call this app now Classic, as well as that we'd recommend using the new generation EX app.

Type of change

• Feature
• Bugfix
• Technical
• Other : README update.

Content

The app is decorated as Element Classic, and its status is clarified.

Motivation and context

Screenshots / GIFs

Tests

• Step 1
• Step 2
• Step ...

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• Changes has been tested on an Android device or Android emulator with API 21
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/element-hq/element-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR
• If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()

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