chore: Replace eventBus with mitt.js [CW-3275] (#9539)

# Replace the deprecated `eventBus` with mitt.js

## Description

Since eventBus and it's respective methods are deprecated and removed
from all future releases of vue, this was blocking us from migrating.
This PR replaces eventBus with
[mitt](https://github.com/developit/mitt). I have created a wrapper
mitt.js to simulate the same old event names so it's backwards
compatible, without making a lot of changes.


Fixes # (issue)

## Type of change

Please delete options that are not relevant.

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

1. Made sure all the places we're listening to bus events are working as
expected.
2. Respective specsf or the events from mitt.


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [x] 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
- [x] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/components/ui/DatePicker/DatePicker.vue

@@ -22,7 +22,7 @@ import {
   setYear,
   isAfter,
 } from 'date-fns';
-
+import { useAlert } from 'dashboard/composables';
 import DatePickerButton from './components/DatePickerButton.vue';
 import CalendarDateInput from './components/CalendarDateInput.vue';
 import CalendarDateRange from './components/CalendarDateRange.vue';
@@ -185,7 +185,7 @@ const updateManualInput = (newDate, calendarType) => {
 };
 
 const handleManualInputError = message => {
-  bus.$emit('newToastMessage', message);
+  useAlert(message);
 };
 
 const resetDatePicker = () => {
@@ -201,7 +201,7 @@ const resetDatePicker = () => {
 
 const emitDateRange = () => {
   if (!isValid(selectedStartDate.value) || !isValid(selectedEndDate.value)) {
-    bus.$emit('newToastMessage', 'Please select a valid time range');
+    useAlert('Please select a valid time range');
   } else {
     showDatePicker.value = false;
     emit('dateRangeChanged', [selectedStartDate.value, selectedEndDate.value]);