Error Tracking: Remove old Postgres backend

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Preamble

Error Tracking with the Postgres backend was disabled in #353639 (closed).

With #361341 (closed) we've enabled Error Tracking with Clickhouse backend behind a feature flag on GitLab.com.

Problem

The old Postgres backend although disabled causes some issues on production and spec failures while consolidating various feature flags.

Proposed solution

Remove the Postgres backend.

Steps:

• Remove ingestion API endpoints - !123139 (merged)
• Remove ErrorTracking::Collector::* and ErrorTracking::CollectErrorService - !123191 (merged)
• Remove the ActiveRecordStrategy from ErrorTracking::ErrorRepository - !123651 (merged)
• Remove related models, theirs specs, drop their tables for:
  • ErrorTracking::Error
  • ErrorTracking::ErrorEvent
• Close issues related to Postgres backend

Risks

Some self-managed users might still use the Postgres backend by enabling the corresponding feature flags. These feature flags were never be meant to be enabled on self-managed instances nor documented.

Using the 🆕 Clickhouse backend is only support on GitLab.com so there is no alternative except for using Sentry backend.