fix(deps): update kotlin to 2.3.0 (#5917)

* fix(deps): update kotlin to 2.3.0

* Cleanup - remove `datetime` compat version

* Fix several lint issues caused by the Kotlin compiler inference working better (checks in nullables, vars, etc.)

* Fix tests by removing mock in `File.readBytes`, it seems like it's no longer allowed. Using a tmp file works well enough.

---------

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: Benoit Marty <benoit@matrix.org>
Co-authored-by: Jorge Martín <jorgem@element.io>

features/rageshake/impl/src/test/kotlin/io/element/android/features/rageshake/impl/reporter/DefaultBugReporterTest.kt

@@ -323,7 +323,7 @@ class DefaultBugReporterTest {
                 while (part != null) {
                     part.headers["Content-Disposition"]?.let { contentDisposition ->
                         regex.find(contentDisposition)?.groupValues?.get(1)?.let { name ->
-                            foundValues.put(name, part!!.body.readUtf8())
+                            foundValues.put(name, part.body.readUtf8())
                         }
                     }
                     part = multipartReader.nextPart()