Fix Java version for gradle:7.4.2.

When Gradle version was bumped to 7.0.0, it required Java version 11. In 82b91214 (2022/01/05), .sdkmanrc was adjusted to accommodate. But app/build.gradle.kts was not.

More recent Gradle releases have started to complain:

,----
| > Could not resolve all files for configuration ':classpath'.
|    > Could not resolve com.android.tools.build:gradle:7.4.2.
|      Required by:
|          project :
|       > No matching variant of com.android.tools.build:gradle:7.4.2 was found. The consumer was configured to find a library for use during runtime, compatible with Java 8, packaged as a jar, and its dependencies declared externally, as well as attribute 'org.gradle.plugin
| .api-version' with value '8.0.2' but:
|           - Variant 'apiElements' capability com.android.tools.build:gradle:7.4.2 declares a library, packaged as a jar, and its dependencies declared externally:
|               - Incompatible because this component declares a component for use during compile-time, compatible with Java 11 and the consumer needed a component for use during runtime, compatible with Java 8
|               - Other compatible attribute:
|                   - Doesn't say anything about org.gradle.plugin.api-version (required '8.0.2')
`----

Adjust gradle’s specification to suit.