Remove issue indexes concurrently

What does this MR do and why?

Remove issue indexes concurrently

After scheduling the removal of the indexes for external_key and health_status, and they successfully ran through on gitlab.com, we can now remove the indexes concurrently from the issues table.

Changelog: removed

References

MRs which scheduled the removal:

• Schedule removing health status index (!200195 - merged)
• Schedule health_status on projects index removal (!200325 - merged)
• Schedule external_key index removal (!200058 - merged)

Issues:

• Remove index on external_key (#558770 - closed)
• Remove health_status on project index (#559053 - closed)
• Remove unneeded health status index (#372205 - closed)

Confirmation that they are removed from gitlab.com:

Index	Confirmation
index_issues_on_project_id_and_external_key
idx_issues_on_health_status_not_null
index_issues_on_project_health_status_asc_work_item_type

Screenshots or screen recordings

Before	After

How to set up and validate locally

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