Migrate bot_type column to user_type

Summary

In #202680 (closed) we introduced user_type enum column which should be used to determine unique user type. We can migrate bot_type column to new user_type column as suggested here

Improvements

Single generalized field is used for determining user type, can be used for STI if needed. Other db columns (e.g. ghost) can be migrated too later on.

Risks

Features with higher risk:

• Code review analytics
• Internal Bot related features

Involved components

• db/schema.rb
• app/models/user.rb
• ee/app/ee/models/ee/user.rb

Technical notes

• Data migration is be required for quite big table.
• New indexes and update of old indexes are required.
• Remove old column in separate issue\MR.