Default on :scan_execution_rule_mode feature flag

What does this MR do and why?

Describe in detail what your merge request does and why.

Default on :scan_execution_rule_mode feature flag

• the feature flag has been on in production for several days now without incident

Changelog: changed

Screenshots or screen recordings

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master branch	current branch

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Upload a GitLab Ultimate license
2. Run echo "Feature.remove(:scan_execution_rule_mode)" | rails c
3. Navigate to http://gdk.test:3443/flightjs/Flight/-/security/policies/new?type=scan_execution_policy

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #359883 (closed)

config/feature_flags/development/scan_execution_rule_mode.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/359883
 milestone: '15.2'
 type: development
 group: group::container security
-default_enabled: false
+default_enabled: true