Remove PG version check from AsWithMaterialized

What does this MR do and why?

Prior to PostgreSQL 12, CTE statements had been materialized by default. The behavior changed in PostgreSQL 12 with the introduction of MATERIALIZED keyword and CTE statements required the keyword to be executed separately. To keep the application code compatible with >= PostgreSQL 11, we've been using a custom Arel node AsWithMaterialized that inserts MATERIALIZED depending on the running PG version. Since GitLab 14.0 requires PostgreSQL 12.7, we can remove the version check from AsWithMaterialized.

Resolves #325639 (closed)

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