Added clients + bugfix, cleaned code, updated readme

[DarkCat09]

Now Killergram supports:

• exteraGram
• Ninjagram
• Graph
• MDGram (only adblock, forwarding doesn't work)
• Nullgram (didn't test)

findIfExists calls and null-checks are moved into XposedClass for cleaner and more readable code (see d7bd65e#diff-3248e43 and d7bd65e#diff-42e00b3)

Updated targetSdk to 33
Updated Gradle to 7.4.2

Removed default icons to reduce APK size.
I don't mind if you return them because of Android Studio recommendations.

In Readme:

1. Updated and rearranged clients list and reformatted into a table
2. Added section for unsupported clients
3. The license link was replaced to the official one
4. Removed/appended blank lines for readability