GitHub Actions: build using Java 19 (use toolchain because Gradle 7.5.1 does not support running on Java 19)

buildSrc/src/main/kotlin/flatlaf-toolchain.gradle.kts

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2022 FormDev Software GmbH
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+plugins {
+	java
+}
+
+val toolchainJavaVersion = System.getProperty( "toolchain" )
+if( !toolchainJavaVersion.isNullOrEmpty() ) {
+	java.toolchain {
+		languageVersion.set( JavaLanguageVersion.of( toolchainJavaVersion ) )
+	}
+}