[bug] NetworkField does not work with MySQL/MariaDB

[pandafy]

Running migrations on MySQL (Server version: 8.0.31 MySQL Community Server - GPL) raises following error

django.db.utils.ProgrammingError: (1064, "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'cidr NOT NULL, `description` varchar(100) NOT NULL, `master_subnet_id` char(32) ' at line 1")

The CIDR data type is also not supported on MariaDB.

openwisp-ipam/openwisp_ipam/base/fields.py

Lines 34 to 43 in 956d9d2

	class NetworkField(models.Field):
	empty_strings_allowed = False
	description = 'CIDR type network field'
	def __init__(self, *args, **kwargs):
	kwargs['max_length'] = 43
	super().__init__(*args, **kwargs)
	def db_type(self, connection):
	return 'cidr'

[Manchuker1120]

I also encountered this problem

[Manchuker1120]

(
'subnet',
models.CharField(
db_index=True,
help_text=(
'Subnet in CIDR notation, eg: "10.0.0.0/24" '
'for IPv4 and "fdb6:21b:a477::9f7/64" for IPv6'
),
max_length=43,
)

I can add openwisp_ipam/migrations/0001_initial.py modified to the above content? If I make changes, I can migrate the database without reporting any errors, but what are the issues if I make the changes?