Revises PostgreSQL-version warning message

What does this MR do?

The groupdatabase team was tagged to review Set PostgreSQL 13 as the minimum supported version (!120551 - merged), and I noticed we could improve the messages we're showing to users with too-low PostgreSQL versions. That required changing these files:

• lib/gitlab/config_checker/external_database_checker.rb
• lib/gitlab/database.rb
• locale/gitlab.pot
• spec/lib/gitlab/config_checker/external_database_checker_spec.rb

The end result:

• Old: Database (name) is using PostgreSQL $version, but PostgreSQL %{pg_version_minimum} is required for this version of GitLab. Please upgrade your environment to a supported PostgreSQL version, see %{pg_requirements_url} for details.
• New: Database (name) is using PostgreSQL $version, but this version of GitLab requires PostgreSQL $version. Please upgrade your environment to a supported PostgreSQL version. See $url for details.

(This MR is really UI text, but the documentation template is closest.)

Related issues

• Related to Set PostgreSQL 13 as the minimum supported version (!120551 - merged) where I spotted the need.

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