Refine .helmignore

What does this MR do?

Refine the .helmignore file to skip more files that are not necessary to be within the generated tarball.

Related issues

Related to #3438 (closed)

Checklist

See Definition of done.

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

Testing

### check master
git checkout master
helm dep update
helm package .
tar tf gitlab*.tgz | grep 'gitlab/[^/]*$'

### this branch
git checkout 3438-helmignore
helm dep update
helm package .
tar tf gitlab*.tgz | grep 'gitlab/[^/]*$'

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
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

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 omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.

Closes #3438 (closed)