Add rubocop rule for ignore_column(s) usages

What does this MR do?

Part of #336751 (closed)

Prior to this change, ignore rules for a columns were defined in CE or EE models. But if a model exists in CE and EE, the ignore rule has to be in the CE model due to possible caching issues of the database column. If the model only exists in EE, then it has to be added there.

A new rubocop rule will check for the correct location of the ignore rule.

How to setup and validate locally (strongly suggested)

Example:

1. Add an ignore rule to an EE model that also has a CE model: e.g. add the following code to ee/app/models/ee/project.rb

ignore_columns :suggestion_commit_message, remove_after: '2021-09-22', remove_with: '14.4'
ignore_column :autoclose_referenced_issues, remove_after: '2021-09-22', remove_with: '14.4'

2. Run rubocop for the ee model and see the errors. e.g.

bundle exec rubocop `ee/app/models/ee/project.rb` --only Cop/IgnoredColumns

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team