Overhaul gradle build

- Focus of this commit is to have modern gradle build.
- Migrate most of a plugin configurations from dsl
  into buildSrc.
- This fixes issues with existing docs build.
- Allows to sign files so that we have that part
  done for central in a build.
- We can skip publishing samples.
- We're able to share similar logic for modules
  which are meant for publish or just being samples.
- It's easier to upgrade gradle versions without
  getting various build issues.
- Relates #1143

gradle/java-test-fixtures.gradle

@@ -1,16 +0,0 @@
-apply plugin: 'java-test-fixtures'
-
-compileTestFixturesJava {
-    sourceCompatibility = 17
-    targetCompatibility = 17
-}
-
-eclipse.classpath {
-	file.whenMerged { classpath ->
-		classpath.entries.findAll { entry -> entry instanceof org.gradle.plugins.ide.eclipse.model.ProjectDependency && entry.entryAttributes.test }
-			.each { it.entryAttributes['test'] = 'false' }
-	}
-}
-
-components.java.withVariantsFromConfiguration(configurations.testFixturesApiElements) { skip() }
-components.java.withVariantsFromConfiguration(configurations.testFixturesRuntimeElements) { skip() }

gradle/publications.gradle

@@ -1,50 +0,0 @@
-apply plugin: "maven-publish"
-
-publishing {
-	publications {
-		mavenJava(MavenPublication) {
-			pom {
-				afterEvaluate {
-					name = project.description
-					description = project.description
-				}
-				url = "https://github.com/spring-projects/spring-statemachine"
-				organization {
-					name = "Spring IO"
-					url = "https://spring.io/spring-statemachine"
-				}
-				licenses {
-					license {
-						name = "Apache License, Version 2.0"
-						url = "https://www.apache.org/licenses/LICENSE-2.0"
-						distribution = "repo"
-					}
-				}
-				scm {
-					url = "https://github.com/spring-projects/spring-statemachine"
-					connection = "scm:git:git://github.com/spring-projects/spring-statemachine"
-					developerConnection = "scm:git:git://github.com/spring-projects/spring-statemachine"
-				}
-				developers {
-					developer {
-						id = 'jvalkeal'
-						name = 'Janne Valkealahti'
-						email = 'janne.valkealahti@gmail.com'
-					}
-				}
-				issueManagement {
-					system = "GitHub"
-					url = "https://github.com/spring-projects/spring-statemachine/issues"
-				}
-			}
-			versionMapping {
-				usage('java-api') {
-					fromResolutionResult()
-				}
-				usage('java-runtime') {
-					fromResolutionResult()
-				}
-			}
-		}
-	}
-}