requirements: Require correct django version

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>

docs/requirements-dev-mysql.txt

-django>=1.11,<1.12
+django>=2.2,<2.3
 mysqlclient>=1.3.0,<1.4
 -r requirements-dev.txt

docs/requirements-dev-postgresql.txt

-django>=1.11,<1.12
+django>=2.2,<2.3
 psycopg2>=2.8,<2.9
 -r requirements-dev.txt

docs/requirements-dev-sqlite.txt

-django>=1.11,<1.12
+django>=2.2,<2.3
 -r requirements-dev.txt

docs/requirements-prod-mysql.txt

-django>=2.1,<2.2
+django>=2.2,<2.3
 mysqlclient>=1.3.0,<1.4
 -r requirements-base.txt

docs/requirements-prod-postgres.txt

-django>=2.1,<2.2
+django>=2.2,<2.3
 psycopg2>=2.8,<2.9
 -r requirements-base.txt