Use version qualifier when preparing major stack releases.

[mashhurs]

Description

We are using centralized qualifier for stack releases and this plugin follows the stack.
It would be good enough to apply the same mechanism to release this plugin with version qualifier.
Reference:

• Use centralized version qualifier logstash#16995
• https://storage.googleapis.com/dra-qualifier/{branch}

[yaauie]

The trouble with synchronizing with the stack version qualifier is that rubygems artifacts are supposed to be immutable, so if we publish an artifact with a specific qualifier before the stack artifacts are pushed we have no opportunity to respin our own artifact.

While I am in favor of releasing pre-release 9.x gems, I would prefer for the qualifier to be decoupled from the stack.

[mashhurs]

I do not think we need to think about this for now, closing...