Force Eclipse to use JavaSE-1.8 runtime

Add `EclipseConventions` to force the JavaSE-1.8 runtime to be applied
to the imported projects.

Closes gh-26669

buildSrc/src/main/java/org/springframework/boot/build/ConventionsPlugin.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -47,6 +47,7 @@ public class ConventionsPlugin implements Plugin<Project> {
 		new MavenPublishingConventions().apply(project);
 		new AsciidoctorConventions().apply(project);
 		new KotlinConventions().apply(project);
+		new EclipseConventions().apply(project);
 	}
 
 }

buildSrc/src/main/java/org/springframework/boot/build/EclipseConventions.java

+/*
+ * Copyright 2012-2021 the original author or authors.
+ *
+ * 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.
+ */
+
+package org.springframework.boot.build;
+
+import org.gradle.api.Project;
+import org.gradle.api.plugins.JavaBasePlugin;
+import org.gradle.plugins.ide.eclipse.EclipsePlugin;
+import org.gradle.plugins.ide.eclipse.model.EclipseModel;
+
+/**
+ * Conventions that are applied in the presence of the {@link JavaBasePlugin} in order to
+ * correctly configure Eclipse.
+ *
+ * @author Phillip Webb
+ */
+class EclipseConventions {
+
+	void apply(Project project) {
+		project.getPlugins().withType(JavaBasePlugin.class, (java) -> {
+			project.getPlugins().apply(EclipsePlugin.class);
+			EclipseModel eclipse = project.getExtensions().getByType(EclipseModel.class);
+			eclipse.getJdt().setJavaRuntimeName("JavaSE-1.8");
+		});
+	}
+
+}