Add a Rule to Check version of Database (MySQL / Postgres / SQLite)

[kaxil]

Many users are not sure about the requirements of the DB versions mentioned in https://github.com/apache/airflow#requirements and run into different errors.

We should have a "Check / Rule" in our Upgrade Check Plugin to warn users if they use a version that will be not compatible with 2.0.0

This issue is part of #8765

How to guide

To implement a new rule, create a class that inherits from airflow.upgrade.rules.base_rule.BaseRule.
It will be auto-registered and used by airflow upgrade-check command. The custom rule class has to have title,
description properties and should implement check method which returns a list of error messages in case of
incompatibility.

For example:

airflow/airflow/upgrade/rules/conn_type_is_not_nullable.py

Lines 25 to 42 in ea36166

	class ConnTypeIsNotNullableRule(BaseRule):
	title = "Connection.conn_type is not nullable"
	description = """\
	The `conn_type` column in the `connection` table must contain content. Previously, this rule was \
	enforced by application logic, but was not enforced by the database schema.
	If you made any modifications to the table directly, make sure you don't have null in the conn_type column.\
	"""
	@provide_session
	def check(self, session=None):
	invalid_connections = session.query(Connection).filter(Connection.conn_type.is_(None))
	return (
	'Connection<id={}", conn_id={}> have empty conn_type field.'.format(conn.id, conn.conn_id)
	for conn in invalid_connections
	)

[kaxil]

cc @Dr-Denzy

[Dr-Denzy]

I will take this on right away.

[kaxil]

Closed by #13955