Put up notice about EOL of LB3 on relevant pages and packages

[hacksparrow]

In a follow up PR of https://github.com/strongloop/loopback-next/pull/6967, we should inform our users that LB3 is not supported anymore. Some of them end up on https://loopback.io/doc/en/lb4/migration-mounting-lb3app.html and assume that's the way to keep using their existing LB3 apps.

I identified the following pages where the notice must be put.

• https://github.com/strongloop/loopback-next/tree/master/packages/booter-lb3app
• https://github.com/strongloop/loopback-next/tree/master/examples/lb3-application
• https://loopback.io/doc/en/lb4/migration-overview.html
• https://loopback.io/doc/en/lb4/migration-mounting-lb3app.html
• https://loopback.io/doc/en/lb3/index.html

[bajtos]

@hacksparrow I think this issue is a duplicate of #6957. Can you please move the list of places you identified to the check-list in #6957 and close this issue?

[hacksparrow]

Closed on favor of https://github.com/strongloop/loopback-next/issues/6957.