Improve adjustment service stability

Gal Katzrequested to merge
gkatz_replace_left_join_in_namespace_stats_adjustment_service into master

What does this MR do and why?

Improve adjustment service stability by replacing usage of LEFT_JOIN with INNER JOIN to make sure we are not trying to insert null value to namespace_id.

Changelog: changed
EE: true

Relates to https://gitlab.com/gitlab-com/request-for-help/-/issues/2784#note_2518330162

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