Enable Rubocop cops that check access modifiers
## What does this MR do?
This MR enables Rubocop cops that detect methods that should be restricted but are the part of public API because of access modifiers used improperly.
This also fixes existing offenses.
## Why was this MR needed?
Some method in our codebase are public instead of being private because it is sometimes difficult to get it right without static analysis.
## What are the relevant issue numbers?
See #17478
Closes #17372
See merge request !5014
This enables following cops:
Check for useless access modifiers
Lint/UselessAccessModifier
Checks for attempts to use `private` or `protected` to set the
visibility of a class method, which does not work.
Lint/IneffectiveAccessModifier
This also disables two false possitives in concerns.
