Add amcheck extension to the doc

What does this MR do?

This change updates the documentation to add a new required database extension called amcheck to the list of PostgreSQL extensions that must be installed. Previously, only two extensions (pg_trgm and btree_gist) were required, but now three extensions are needed for the database to work properly with GitLab.

Related issues

• gitlab-org/gitlab-environment-toolkit!1655 (comment 2729720021)
• Add amcheck to postgresql extension doc (gitlab-org/gitlab!203824 - merged)

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• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
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