Deprecate com.vaadin:vaadin and com.vaadin:vaadin-core

[mshabarov]

com.vaadin:vaadin and com.vaadin:vaadin-core are deprecated (in documentation and log a warning message in runtime), but not removed.

The recommended way is to use vaadin-core-spring-boot-starter, vaadin-flow-core-spring-boot-starter or standalone flow/components/add-on dependencies.

The deprecation is proposed because we want to encourage users to use a better way in the form of single starter artifact or more granular smaller artifacts.

[web-padawan]

Related to that, should we also deprecate and eventually remove the JS repos as they are not up to date?

• https://github.com/vaadin/vaadin-core
• https://github.com/vaadin/vaadin