feat: Replace alertMixin usage with useAlert (#9793)

# Pull Request Template

## Description

This PR will replace the usage of `alertMixin` from the code base with
the `useAlert` composable.

Fixes
https://linear.app/chatwoot/issue/CW-3462/replace-alertmixin-usage-with-usealert

## Type of change

- [x] Breaking change (fix or feature that would cause existing
functionality not to work as expected)

## How Has This Been Tested?

Please refer this issue description

https://linear.app/chatwoot/issue/CW-3462/replace-alertmixin-usage-with-usealert


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

app/javascript/dashboard/components/layout/sidebarComponents/SecondaryNavItem.vue

@@ -94,8 +94,7 @@
 
 <script>
 import { mapGetters } from 'vuex';
-
-import adminMixin from '../../../mixins/isAdmin';
+import { useAdmin } from 'dashboard/composables/useAdmin';
 import configMixin from 'shared/mixins/configMixin';
 import {
   getInboxClassByType,
@@ -111,13 +110,19 @@ import Policy from '../../policy.vue';
 
 export default {
   components: { SecondaryChildNavItem, Policy },
-  mixins: [adminMixin, configMixin],
+  mixins: [configMixin],
   props: {
     menuItem: {
       type: Object,
       default: () => ({}),
     },
   },
+  setup() {
+    const { isAdmin } = useAdmin();
+    return {
+      isAdmin,
+    };
+  },
   computed: {
     ...mapGetters({
       activeInbox: 'getSelectedInbox',