Improve screenshot testing with ComposablePreviewScanner (#3125)

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

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

app/build.gradle.kts

@@ -31,7 +31,6 @@ plugins {
     id("io.element.android-compose-application")
     alias(libs.plugins.kotlin.android)
     alias(libs.plugins.anvil)
-    alias(libs.plugins.ksp)
     alias(libs.plugins.kapt)
     // When using precompiled plugins, we need to apply the firebase plugin like this
     id(libs.plugins.firebaseAppDistribution.get().pluginId)
@@ -283,6 +282,5 @@ dependencies {
     testImplementation(libs.test.turbine)
     testImplementation(projects.libraries.matrix.test)
 
-    ksp(libs.showkase.processor)
     koverDependencies()
 }