Only allow validation when `allow_editing_commit_messages` is changed to true in `project_setting`

What does this MR do?

related to #328639 (closed)

There is a validation error happening ActiveRecord::RecordInvalid: Validation failed: Allow editing commit messages can't be enabled because signed commits are required for this project after the validation is introduced here. We discovered while we were discussion about this in TI refinement session, this validation is only required when allow_editing_commit_messages is enabled, otherwise we can skip this. If continue to do this for attribute update, this will throw an error like https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/security/store_reports_service.rb#L34

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.