Fix key name in image tag spec helper

What does this MR do?

Fix key name in image tag spec helper

Fixes the reference from:

'global.gitlab_version'

to:

'global.gitlabVersion'

in the 'fetch_default_gitlab_version' spec helper used in the image
tag tests to match the syntax from values.yaml.

Follow-up to https://gitlab.com/gitlab-org/charts/gitlab/-/merge_requests/2991

Related to #4262 (closed).

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
• 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.