[Hello World Tutorial] Bump Php alpine docker image version to 8.4.1-alpine3.21 (#7751)

chore: Update the value of the php docker image for pipelines in the ...

... 'Hello World!' tutorial

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content/doc/pipeline/tour/hello-world.adoc

@@ -164,7 +164,7 @@ node {
 // Declarative //
 /* Requires the Docker Pipeline plugin */
 pipeline {
-    agent { docker { image 'php:8.4.1-alpine3.20' } }
+    agent { docker { image 'php:8.4.1-alpine3.21' } }
     stages {
         stage('build') {
             steps {