Move Antora Tasks into DocsPlugin

All Spring projects using io.spring.convention.docs are also using Antora,
so these tasks belong in the convention rather than each project's build script.

Issue gh-18968

docs/spring-security-docs.gradle

@@ -20,22 +20,11 @@ antora {
 	]
 }
 
-tasks.register("syncAntoraAttachments", Sync) {
-	group = 'Documentation'
-	description = 'Syncs the Antora attachments'
-	from project.provider( { project.tasks.api.outputs } )
-	into project.layout.buildDirectory.dir('generated-antora-resources/modules/ROOT/assets/attachments/api/java')
-}
-
 tasks.named("generateAntoraYml") {
 	asciidocAttributes = project.provider( { generateAttributes() } )
 	asciidocAttributes.putAll(providers.provider( { resolvedVersions(project.configurations.testRuntimeClasspath) }))
 }
 
-tasks.register("generateAntoraResources") {
-	dependsOn 'generateAntoraYml', 'syncAntoraAttachments'
-}
-
 dependencies {
 	testImplementation platform(project(':spring-security-dependencies'))
 	testImplementation project(':spring-security-config')