Upgrade to Spring Asciidoctor Extensions 0.4.0.RELEASE

0.4.0 provides built-in support for remembering a user's selections
using local storage. This replaces the custom switch language
JavaScript.

The selection is stored using a key derived from the
options that were available. Concretely, when the options are Java or
Kotlin, the local storage key is java-kotlin. Similarly, if the
choices were Java, Kotlin, and XML, the key would be java-kotlin-xml.

Given local storage's domain and protocol scoping, the nature of the
key that's used for storage will allow a user's selections to be
applied across all documentation hosted on https://docs.spring.io that
offer the same options.

Closes spring-projectsgh-24481

gradle/docs.gradle

@@ -3,7 +3,16 @@ configurations {
 }
 
 dependencies {
-	asciidoctorExt("io.spring.asciidoctor:spring-asciidoctor-extensions:0.2.0.RELEASE")
+	asciidoctorExt("io.spring.asciidoctor:spring-asciidoctor-extensions-block-switch:0.4.0.RELEASE")
+}
+
+repositories {
+	maven {
+		url "https://repo.spring.io/release"
+		mavenContent {
+			group "io.spring.asciidoctor"
+		}
+	}
 }
 
 /**

src/docs/asciidoc/docinfo-footer.html

@@ -1,3 +1,2 @@
 <script type="text/javascript" src="js/tocbot/tocbot.min.js"></script>
 <script type="text/javascript" src="js/toc.js"></script>
-<script type="text/javascript" src="js/switch-language.js"></script>