Update quickstart.md

[akgerber]

Description of the change: replace --plugins=go/v4-alpha with --plugins=go/v4 to reflect "Move go/v4 plugin from alpha to stable " in v1.33 https://sdk.operatorframework.io/docs/upgrading-sdk-version/v1.33.0/

Motivation for the change: docs are out of date

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• [ ] Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• [ ] Add or update relevant sections of the docs website in website/content/en/docs

[acornett21]

/ok-to-test

[OchiengEd]

@akgerber Could you see that your commit is properly signed. See the checks for more information

[jberkhahn]

still need you to sign your commit so this can be merged

[acornett21]

Closing as duplicate.