Deprecate Drupal\Core\Database\Connection::makeSequenceName(), make it internal to PostgreSQL

Problem/Motivation

The method Drupal\Core\Database\Connection::makeSequenceName() is only used internally the PgSQL driver, but it's exposed as a public method for the entire Database API.

Proposed resolution

1. deprecate Drupal\Core\Database\Connection::makeSequenceName()
2. mark the PgSql driver implementation @internal or even better a protected method of PgSql's Schema class.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet