Fix loading state for split button

What does this MR do and why?

This MR fixes the loading state being ignored in the finding modal.

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Fork https://gitlab.com/gitlab-examples/security/security-reports/
2. Create an MR
3. Visit the MR page
4. Find the container scanning report and click on a finding
5. Click on the Resolve with MR (if you have :refactor_security_extension feature flag turned on it may not work, see the next step in that case)

You can also apply the following patch to force displaying the loading state:

diff --git a/ee/app/assets/javascripts/vue_shared/security_reports/components/modal_footer.vue b/ee/app/assets/javascripts/vue_shared/security_reports/components/modal_footer.vue
index 5dc9440c49cb..5f13f6a25d32 100644
--- a/ee/app/assets/javascripts/vue_shared/security_reports/components/modal_footer.vue
+++ b/ee/app/assets/javascripts/vue_shared/security_reports/components/modal_footer.vue
@@ -83,7 +83,7 @@ export default {
       const MRButton = {
         name: s__('ciReport|Resolve with merge request'),
         tagline: s__('ciReport|Automatically apply the patch in a new branch'),
-        loading: this.isCreatingMergeRequest,
+        loading: true,
         action: 'createMergeRequest',
       };
       const DownloadButton = {

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 #390529 (closed)