Fix issue_metrics index creation error

What does this MR do and why?

This MR fixes the schema inconsistency error on self-managed instances related to the issue_metrics table. In some cases the issue_metrics.first_mentioned_in_commit_at column can have the following type: timestamp with time zone. According to the structure.sql the schema should be timestamp without time zone.

Fix: detect the column type and add the index accordingly.

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Related to #343694 (closed)