Allow updating controls when requirement has five controls

What does this MR do and why?

Fixes saving a compliance framework when a requirement has five controls.

The control count validation (ComplianceRequirementsControl#controls_count_per_requirement) also counted the control being saved. So when a requirement had five controls, updating any existing control failed with "Compliance requirement cannot have more than 5 controls". This blocked the whole framework update, even when the requirement wasn't changed.

The validation now excludes the current record from the count. Adding a 6th control is still blocked.

Resolves #630360 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to a group's Secure > Compliance center > Frameworks and create a framework with a requirement that has 5 controls.
2. Edit the requirement and remove one control, or replace "At least two approvals" with "At least one approval".
3. Click Update framework.
4. Before this change, it fails with Compliance requirement cannot have more than 5 controls. With this change, it saves.

Specs:

bin/rspec ee/spec/models/compliance_management/compliance_framework/compliance_requirements_control_spec.rb \
  ee/spec/services/compliance_management/compliance_framework/compliance_requirements/update_service_spec.rb

MR acceptance checklist

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Related to #630360 (closed)