Remove separate dependency management for netty-tcnative in favor of the entry in Netty’s bom

[oburgosm]

Since netty added tcnative to its BOM, it's no longer necessary to manage netty-tcnative in spring-boot-dependencies

[bclozel]

This is a breaking change, since this means removing the netty-tcnative.version version property.
Tentatively scheduling this for 3.0.x.

[wilkinsona]

I think this is the sort of breaking change that would be acceptable in a 2.x release as it's fairly easy to define your own dependency management for tcnative if you want to use a version that's out of line with Netty's.

[snicoll]

I've noticed at least once that the netty bom didn't match with the latest tcnative version (as they found out there was an issue with it and released a new version without releasing a new version of netty). I am wondering if keeping control wouldn't be so bad considering bomr has been doing a great job thus far.

[oburgosm]

Please also note that currently spring boot is not managing all netty-tcnative artifacts. So artifacts not managed by Spring Boot but managed by netty bom could be in conflict with artifacts managed by Spring Boot.