Let's remove api from the io.netty5.buffer.api package name

[chrisvest]

The api part is from before we differentiated the Netty 5 packages with io.netty5, so it was used to keep the old and new APIs separate.

However, now that the Netty 5 packages are differentiated from Netty 4, and we've removed ByteBuf from Netty 5, there's no longer a need to keep this extra level. It looks out of place.

On the other hand, changing it is going to impact a lot of code, including all of the new contrib repositories.

Thoughts?

[normanmaurer]

Now is the time

[chrisvest]

Let's goo! #12792