create column with default rework, expicit constraints drop before drop table and column and bug fixes

• changed ADD COLUMN DEFAULT NULL to safe operation for code default
• changed ADD COLUMN DEFAULT NOT NULL to safe operation for db_default in django 5.0+
• added ZERO_DOWNTIME_MIGRATIONS_KEEP_DEFAULT settings and changed ADD COLUMN DEFAULT NOT NULL with this settings to safe operation for django<5.0
• added ZERO_DOWNTIME_MIGRATIONS_EXPLICIT_CONSTRAINTS_DROP settings and added dropping constraints and indexes before drop column or table
• fixed sqlmigrate in idempotent mode
• fixed creation unique constraint with include parameter
• fixed idempotent mode tests
• updated unsafe migrations links to documentation
• updated patched code to latest django version
• updated test image to ubuntu 24.04
• improved README