Exclude unnecessary files in gitlab-grpc gem

What does this MR do?

When the gitlab-grpc gem gets merged into GitLab via gitlab!82766 (closed), the installed libraries take over 300 MB of source data and compiled objects. These files are not needed for Omnibus, so exclude them as we do with the grpc gem.

Related issues

Part of #6687 (closed)

Test build: https://dev.gitlab.org/gitlab/omnibus-gitlab/-/merge_requests/213

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened