Fix: permission callback for email-reporting EDITABLE route

[mrtori161]

Summary

Addresses issue:

• Fix permission callback for email-reporting EDITABLE route #12341

Relevant technical choices

The core/site/data/email-reporting EDITABLE route's permission_callback is changed from $can_access to $can_manage, matching every other write route in REST_Email_Reporting_Controller:

- 'permission_callback' => $can_access,                                                                                                                                                                                                     
+ 'permission_callback' => $can_manage,                                                                                                                                                                                                       
                                                                                                                                                                                                                                            
The $can_access closure checks VIEW_SPLASH / VIEW_DASHBOARD, which shared dashboard users (Subscribers, Contributors) satisfy. The $can_manage closure checks MANAGE_OPTIONS, restricting modification of the global email reporting setting  
to administrators — consistent with email-reporting-invite-user, email-reporting-eligible-subscribers, and email-reporting-errors.
                                                                                                                                                                                                                                              
PR Author Checklist                                                                                                                                                                                                                         
                                                                                                                                                                                                                                            
- My code is tested and passes existing unit tests.
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- I have added a QA Brief on the issue linked above.                                                                                                                                                                                          
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