Pin javalin to managed version for docs

It seems that the docs use `latest:release` for javalin:
https://github.com/micrometer-metrics/micrometer/blob/e122b103fb9a00f4baca0f1ed2f3833161fca31f/docs/build.gradle#L39
This makes our build non-reproducible and also the build is failing
right now because of this since
io.javalin:javalin:6.0.0-beta.1 was published to Maven Central.

Closes micrometer-metricsgh-4299

docs/build.gradle

@@ -36,7 +36,7 @@ dependencies {
     testImplementation 'io.projectreactor:reactor-test'
     testImplementation 'io.netty:netty-transport'
     testImplementation 'org.apache.httpcomponents.client5:httpclient5'
-    testImplementation 'io.javalin:javalin:latest.release'
+    testImplementation libs.javalin
 }
 
 antora {