Bump rxkotlin from 2.3.0 to 2.4.0

Bumps [rxkotlin](https://github.com/ReactiveX/RxKotlin) from 2.3.0 to 2.4.0.
- [Release notes](https://github.com/ReactiveX/RxKotlin/releases)
- [Commits](https://github.com/ReactiveX/RxKotlin/compare/2.3.0...2.4.0)

Signed-off-by: dependabot[bot] <support@github.com>

attachment-viewer/build.gradle

@@ -63,7 +63,7 @@ android {
 dependencies {
     implementation 'com.github.chrisbanes:PhotoView:2.1.4'
 
-    implementation 'io.reactivex.rxjava2:rxkotlin:2.3.0'
+    implementation 'io.reactivex.rxjava2:rxkotlin:2.4.0'
     implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'
 
     implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"

matrix-sdk-android-rx/build.gradle

@@ -36,7 +36,7 @@ android {
 dependencies {
     implementation project(":matrix-sdk-android")
     implementation 'androidx.appcompat:appcompat:1.2.0'
-    implementation 'io.reactivex.rxjava2:rxkotlin:2.3.0'
+    implementation 'io.reactivex.rxjava2:rxkotlin:2.4.0'
     implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'
     implementation "org.jetbrains.kotlinx:kotlinx-coroutines-rx2:$kotlin_coroutines_version"
 

vector/build.gradle

@@ -341,7 +341,7 @@ dependencies {
     implementation 'com.googlecode.libphonenumber:libphonenumber:8.12.18'
 
     // rx
-    implementation 'io.reactivex.rxjava2:rxkotlin:2.3.0'
+    implementation 'io.reactivex.rxjava2:rxkotlin:2.4.0'
     implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'
     implementation 'com.jakewharton.rxrelay2:rxrelay:2.1.1'
     // RXBinding