Pin Node.js version to 24.15.0

Prior to this commit, the `antora` Gradle task silently failed to build
the reference documentation, since Antora uses the latest LTS release
for Node.js by default, and the latest LTS apparently does not work for
us.

Author: sbrannen

build.gradle

@@ -6,3 +6,7 @@ plugins {
 antora {
 	options = [clean: true, fetch: true, stacktrace: true]
 }
+
+node {
+	version = '24.15.0'
+}