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>

features/share/impl/build.gradle.kts

@@ -16,7 +16,6 @@
 
 plugins {
     id("io.element.android-compose-library")
-    alias(libs.plugins.ksp)
     alias(libs.plugins.anvil)
     id("kotlin-parcelize")
 }
@@ -65,6 +64,4 @@ dependencies {
     testImplementation(projects.libraries.mediaupload.test)
     testImplementation(projects.tests.testutils)
     testReleaseImplementation(libs.androidx.compose.ui.test.manifest)
-
-    ksp(libs.showkase.processor)
 }