• Remove deprecated constants and methods.

For all db drivers

• Enable BC tests
  https://github.com/yiisoft/db/blob/master/.github/workflows/bc.yml_
• Enable Rector tests
  https://github.com/yiisoft/db-mssql/blob/master/.github/workflows/rector.yml_
• Change dependency ofdb to^2.0 incomposer.json
  https://github.com/yiisoft/db-pgsql/blob/3dab24db501c6dc06614eac4554b963bbb4e7b56/composer.json#L28