Use new VaadinWebSecurity for Vaadin Spring Security

[mshabarov]

Update Securing Spring Boot Applications article with a new VaadinWebSecurity API introduced as a replacement for deprecated WebSecurityConfigurerAdapter here:

• code example section https://vaadin.com/docs/latest/security/enabling-security/#security-configuration-class
• note that VaadinWebSecurityConfigurerAdapter can still be used but would be excluded in the later versions
• prepare a branch in https://github.com/vaadin/flow-crm-tutorial/blob/v23/src/main/java/com/example/application/security/SecurityConfig.java to update the code examples there as well.

[tarekoraby]

When we change the docs to use VaadinWebSecurity, let's add a small note mentioning the old VaadinWebSecurityConfigurerAdapter for 23.1 users. We can remove this note after 23.1 is no longer supported.