fix: postpone frontend bootstrap file generation (#20947)

Generation of vaadin.ts bootstrap file allows developers to modify the contents
by providing implementations of the TypeScriptBootstrapModifier interface.
However, this task is executed early and the modifier may look for files that
are not yet generated or that can be deleted by subsequent tasks.
This change move the generation of the bootstrap file just before building
the dev bundle so that all the runtime generated files are available to the
modifiers.

Fixes #20946

Author: mcollovati

flow-server/src/main/java/com/vaadin/flow/server/frontend/NodeTasks.java

@@ -69,7 +69,6 @@ public class NodeTasks implements FallibleCommand {
             TaskGenerateTsConfig.class,
             TaskGenerateTsDefinitions.class,
             TaskGenerateServiceWorker.class,
-            TaskGenerateBootstrap.class,
             TaskGenerateWebComponentHtml.class,
             TaskGenerateWebComponentBootstrap.class,
             TaskGenerateFeatureFlags.class,
@@ -86,6 +85,7 @@ public class NodeTasks implements FallibleCommand {
             TaskUpdateImports.class,
             TaskUpdateThemeImport.class,
             TaskCopyTemplateFiles.class,
+            TaskGenerateBootstrap.class,
             TaskRunDevBundleBuild.class,
             TaskPrepareProdBundle.class,
             TaskCleanFrontendFiles.class,