KT-34795 Use maven(...) instead of maven { setUrl(...) } for gradle scripts generation

- Use new syntax in tests, but not in actual gradle files
- Reuse public `toKotlinRepositorySnippet` instead of private one
- ^KT-34795 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/NewMultiplatformIT.kt

@@ -139,7 +139,7 @@ class NewMultiplatformIT : BaseGradleIT() {
 
         with(appProject) {
             setupWorkingDir()
-            gradleBuildScript().appendText("\nrepositories { maven { setUrl(\"$libLocalRepoUri\") } }")
+            gradleBuildScript().appendText("\nrepositories { maven(\"$libLocalRepoUri\") }")
 
             fun CompiledProject.checkProgramCompilationCommandLine(check: (String) -> Unit) {
                 output.lineSequence().filter {