fix: set load-on-startup for automatically registered Vaadin servlets

Description

When using Vite, DevModeInitializer blocks dev-server startup until a VaadinServlet is deployed because it needs to get the servlet path to use. If the container lazily loads servlets, Vite will not start until the first HTTP request for the Vaadin servlet is received. This change sets load-on-startup feature for all Vaadin Servlet registered by the extension, to ensure that Vite is loaded at the startup of the Web application. A warning message is shown for custom Vaadin servlets that do not set loadOnStartup on WebServlet annotation.

Part of vaadin/flow#14479

Type of change

[X] Bugfix
[ ] Feature

Checklist

[X] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
[X] I have added a description following the guideline.
[X] The issue is created in the corresponding repository and I have referenced it.
[ ] I have added tests to ensure my change is effective and works as intended.
[X] New and existing tests are passing locally with my change.
[X] I have performed self-review and corrected misspellings.