Migrate DRA GCS publishing from Release manager to dra-prep-plugin

[ninalee12]

Summary of your changes

Replaces the Release Manager Docker step with elastic/dra-prep-buildkite-plugin +
a trigger to unified-release-dra-processing. Part of the platform-wide Release
Manager deprecation

Related Issues

• https://github.com/elastic/platform-engineering-productivity/issues/3239

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary README/documentation (if appropriate)

Test

• run a snapshot DRA build
• run a staging DRA build

[mergify]

This pull request does not have a backport label. Could you fix it @ninalee12? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.