Update `minSdkVersion` (from `21` to `24`)

[ParaskP7]

This PR upgrades Login to minSdkVersion to 24. This has been unblocked because:

• WPAndroid was already on minSdkVersion = 24, for some time now (see here).
• WCAndroid has been recently upgraded to minSdkVersion = 24 as well (see here).

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As part of this minSdkVersion = 24 upgrade the below changes were also applied in order to fix any additional warnings that this change brought-up:

Warnings Resolution List:

1. Resolve obsolete sdk int lint warning for res values v23.
2. Remove unnecessary duplicate v27 theme login flow config.

Dependency Config List:

1. Rename kotlin ktx to androidx core version.
2. Extract all dependency version to root build gradle.
3. Add androidx prefix to app compat version.
4. Move androidx app compat/core and dagger versions to main group.

Dependency Update List:

1. Update robolectric to 4.9.

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PS: @ovitrif @atorresveiga I added you as the main reviewers, that is, in addition to the @wordpress-mobile/apps-infrastructure team itself, but randomly, since I just wanted someone from both, the WordPress and Woo Commerce teams, to sign-off on that change for WPAndroid and WCAndroid alike.

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Testing instructions

• There is nothing much to test here.
• Verifying that all the CI checks are successful should be enough.
• However, if you want to be thorough about reviewing this change, you could:
  • Quickly smoke test, either the WordPress or Woo Commerce apps, with this version of Login, and see if it works as expected.
  • Try and install, either the WordPress or Woo Commerce apps, with this version of Login, on Android 5 (API 21/22) and Android 6 (API 23) devices and verify that you can't (see API Levels).

[ParaskP7]

👋 @ovitrif !

Thank you so much for reviewing, testing and catching this issue, you are a hero! 🥇

Overall the changes LGTM, I've left a comment for an issue I've noticed on the status bar color in WP-Android.

Replied here. 🙇 🥇 🙏

I understand from https://github.com/wordpress-mobile/WordPress-FluxC-Android/pull/2563#discussion_r1020091519 that you may be reviewing & possibly change the approach with gradle.ext... versions in settings.gradle.

Yes, exactly, thus, please let do the merging part, that is, when I figure-out how to best progress with the gradle.ext thingie... 🤔

Please let me know when that's decided, as well as the resolution for the status bar color is handled, so I can give this another pass.

It is now ready for another pass, please take another look when possible, no rush! 💯

[ParaskP7]

👋 @ovitrif !

I've tested again the login flow in WP Android and the issue with the statusBar is resolved as expected 🥇 .

A BIG thank you Ovi! 🙇 🙇 🙇

[ParaskP7]

Thank you so much for the review and testing @atorresveiga , you are awesome! 🙇 🥇 💯