Cayenne 5 to require Java 21

Another heads up. I think we need to bump up Cayenne 5.0 to Java 21 from =
11:

1. J11 is quickly becoming a problem (e.g., we are building an MCP =
server, and MCP SDK requires J17)
2. The final release is still some time away (a year or more). By then =
J11 will even more of an ancient history
3. 4.2.x still works fine on Java 8, 11 or newer

Of course everyone's feedback is welcome.=20

Andrus