SQLAlchemy typing and drop deprecated mypy plugin

[chaen]

we currently have #type: ignore comments in places like that:

diracx/diracx-db/src/diracx/db/sql/auth/db.py

Line 97 in a37ff0d

for _ in range(DeviceFlows.user_code.type.length) # type: ignore

This is because we need to specify the types at the table level

diracx/diracx-db/src/diracx/db/sql/auth/schema.py

Line 44 in a37ff0d

user_code = Column("UserCode", String(USER_CODE_LENGTH), primary_key=True)

Should be changed to something like

user_code: Mapped[str] = mapped_column("UserCode", String(USER_CODE_LENGTH), primary_key=True)

Conversion guide:

https://docs.sqlalchemy.org/en/20/changelog/whatsnew_20.html#new-typing-support-in-core-and-orm-stubs-extensions-no-longer-used

Also the mypy plugin is no longer supported and should be dropped: #438