Improve error message when omnibus_gitconfig is not set properly

What does this MR do?

Some users have configured omnibus_gitconfig['system'] improperly, causing this obscure error:

NoMethodError: undefined method `lstrip' for nil:NilClass
/opt/gitlab/embedded/cookbooks/cache/cookbooks/gitaly/libraries/gitaly.rb:93:in `block (2 levels) in parse_gitconfig'
/opt/gitlab/embedded/cookbooks/cache/cookbooks/gitaly/libraries/gitaly.rb:91:in `map'
/opt/gitlab/embedded/cookbooks/cache/cookbooks/gitaly/libraries/gitaly.rb:91:in `block in parse_gitconfig'

This commit ensures each config line has a proper key=value form and displays a clearer message. For example:

RuntimeError
------------
Invalid entry detected in omnibus_gitconfig['system']: 'fsckObjects' should be in the form key=value

Related issues

Relates to #6986 (closed)

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
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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