Rename pullsecrets template to gitlab.image.pullSecrets

What does this MR do?

This MR is a follow-up of !2120 (merged) and it should be merged after that.

It simply renames pullsecrets template to gitlab.image.pullSecrets to match other gitlab.image.* templates.

Related issues

Related to #1617 (closed)

Further work

Introduce global.image.pullSecrets attribute

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

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