Deprecate support for Gradle 6

Closes gh-1437

Author: snicoll

initializr-generator-spring/src/main/java/io/spring/initializr/generator/spring/build/gradle/GradleProjectGenerationConfiguration.java

@@ -139,6 +139,7 @@ public GradleBuildProjectContributor gradleKtsBuildProjectContributor(KotlinDslG
 	 */
 	@Configuration
 	@ConditionalOnGradleVersion("6")
+	@Deprecated
 	static class Gradle6ProjectGenerationConfiguration {
 
 		@Bean

initializr-generator-spring/src/test/java/io/spring/initializr/generator/spring/build/gradle/ConditionalOnGradleVersionTests.java

@@ -37,6 +37,7 @@ public class ConditionalOnGradleVersionTests {
 		.withConfiguration(GradleVersionTestConfiguration.class);
 
 	@Test
+	@Deprecated
 	void outcomeWithSpringBoot23() {
 		MutableProjectDescription description = new MutableProjectDescription();
 		description.setPlatformVersion(Version.parse("2.3.10.RELEASE"));
@@ -47,6 +48,7 @@ void outcomeWithSpringBoot23() {
 	}
 
 	@Test
+	@Deprecated
 	void outcomeWithSpringBoot24() {
 		MutableProjectDescription description = new MutableProjectDescription();
 		description.setPlatformVersion(Version.parse("2.4.0"));
@@ -132,17 +134,18 @@ void outcomeWithNoAvailableSpringBootVersion() {
 	@Test
 	void outcomeWithSpringBoot24AndMultipleGenerations() {
 		MutableProjectDescription description = new MutableProjectDescription();
-		description.setPlatformVersion(Version.parse("2.4.0"));
-		this.projectTester.withConfiguration(Gradle6Or7TestConfiguration.class)
+		description.setPlatformVersion(Version.parse("2.7.0"));
+		this.projectTester.withConfiguration(Gradle7Or8TestConfiguration.class)
 			.configure(description,
-					(context) -> assertThat(context).getBeanNames(String.class).containsOnly("gradle6", "gradle6Or7"));
+					(context) -> assertThat(context).getBeanNames(String.class).containsOnly("gradle7", "gradle7Or8"));
 	}
 
 	@Configuration
 	static class GradleVersionTestConfiguration {
 
 		@Bean
 		@ConditionalOnGradleVersion("6")
+		@Deprecated
 		String gradle6() {
 			return "testGradle6";
 		}
@@ -162,12 +165,12 @@ String gradle8() {
 	}
 
 	@Configuration
-	static class Gradle6Or7TestConfiguration {
+	static class Gradle7Or8TestConfiguration {
 
 		@Bean
-		@ConditionalOnGradleVersion({ "6", "7" })
-		String gradle6Or7() {
-			return "testGradle6Or7";
+		@ConditionalOnGradleVersion({ "7", "8" })
+		String gradle7Or8() {
+			return "testGradle7Or8";
 		}
 
 	}