Improve screenshot testing with ComposablePreviewScanner (#3125)

* Use ComposablePreviewScanner to rework how screenshot testing works
* Add test sharding
* Update screenshots
* Fixes for Element Gallery

---------

Co-authored-by: ElementBot <benoitm+elementbot@element.io>
Co-authored-by: Benoit Marty <benoit@matrix.org>

appnav/build.gradle.kts

@@ -21,7 +21,6 @@ import extension.allFeaturesApi
 plugins {
     id("io.element.android-compose-library")
     alias(libs.plugins.anvil)
-    alias(libs.plugins.ksp)
     alias(libs.plugins.kapt)
     id("kotlin-parcelize")
 }
@@ -78,6 +77,4 @@ dependencies {
     testImplementation(projects.services.analytics.test)
     testImplementation(libs.test.appyx.junit)
     testImplementation(libs.test.arch.core)
-
-    ksp(libs.showkase.processor)
 }