docs: Gitaly logging format default is invalid

Gitaly doesn't support the string 'default' as value without warning the user. Supported values are either the empty string, or 'json'. This change updates the documentation to be valid again.

Closes gitlab#212826 (closed)

changed milestone to %12.10

added groupgitaly label

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merged

mentioned in commit 4556d3b6

mentioned in issue gitaly#2666 (closed)

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added workflowcanary label and removed workflowstaging label

added workflowproduction label and removed workflowcanary label

This merge request has been deployed to the pre.gitlab.com environment, and will be included in the upcoming self-managed GitLab 12.10.0 release.

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added workflowproduction label and removed workflowstaging-ref label