Mysql5.7 is not supported any more, remove support for Mysql5.7

[email protected] is not supported any more upstream and also not in cf-deployment. PR validation could be sped up by removing the checks for MySQL5.7. Extra lines in migrations, that are needed to be compliant with Mysql5.7, would not be necessary (e.g. removing unique constraint in MySQL 5.7 does not include removing the corresponding index, in higher versions it does).

Steps to Reproduce

Write a migration to drop a unique constraint, like

Sequel.migration do
  up do
    if self.class.name.match?(/mysql/i)
      alter_table :quota_definitions do
        drop_constraint :name, if_exists: true
      end
    elsif self.class.name.match?(/postgres/i)
      alter_table :quota_definitions do
        drop_constraint :quota_definitions_name_key, if_exists: true
      end
    end
  end

  down do
    if self.class.name.match?(/mysql/i)
      alter_table :quota_definitions do
        add_unique_constraint :name, name: :name, if_not_exists: true
      end
    elsif self.class.name.match?(/postgres/i)
      alter_table :quota_definitions do
        add_unique_constraint :name, name: :quota_definitions_name_key, if_not_exists: true
      end
    end
  end
end

and open a PR. It will fail with Mysql2::Error: Duplicate key name 'name' for MySQL5.7.

Expected result

PR validation runs without check for MySQL5.7, same for Backwards Compatibility Unit Tests.

Current result

Migrations and migration specs need to be compliant for MySQL5.7

Possible Fix

Remove support for MySQL5.7 and the MySQL5.7 PR validation checks.