feat: bump default kotlin to version 1.7.21 (#1594)

The rationale is based on the Gradle Compatibility Matrix: https://docs.gradle.org/current/userguide/compatibility.html

1.7.21 is the current latest version of the 1.7 Kotlin release.

Author: breautek

framework/cdv-gradle-config-defaults.json

@@ -5,7 +5,7 @@
     "GRADLE_VERSION": "7.6",
     "MIN_BUILD_TOOLS_VERSION": "33.0.2",
     "AGP_VERSION": "7.4.2",
-    "KOTLIN_VERSION": "1.5.21",
+    "KOTLIN_VERSION": "1.7.21",
     "ANDROIDX_APP_COMPAT_VERSION": "1.6.1",
     "ANDROIDX_WEBKIT_VERSION": "1.6.0",
     "ANDROIDX_CORE_SPLASHSCREEN_VERSION": "1.0.0",