Configure proper JDK toolchain version for 'proto-test-models'

Main projects, while using default JDK toolchain 11, are configured to
build with JDK release 8. After recent changes in KGP attributes checks
for JVM compatibility is started working and complain about incompatible
JVM target version for 'proto-test-models' project. To avoid this I
downgraded JDK toolchain version in this project to 8.

Author: Tapchicoma

formats/protobuf/proto-test-model/build.gradle.kts

@@ -17,7 +17,8 @@ protobuf {
     }
 }
 
-java.toolchain.languageVersion.set(JavaLanguageVersion.of(jdkToolchainVersion))
+// Toolchain version should be the same as JDK release in source-sets-convention
+java.toolchain.languageVersion.set(JavaLanguageVersion.of(8))
 
 tasks.clean {
     delete(protobuf.protobuf.generatedFilesBaseDir)
@@ -31,4 +32,4 @@ sourceSets.main {
 
 dependencies {
     api(libs.protobuf.java)
-}
+}