[Issue] Only Update Indexer Mode When Required

[m2-assistant]

This issue is automatically created based on existing pull request: #36422: Only Update Indexer Mode When Required

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Description (*)

Updates the indexer mode change mass actions to only change modes when required i.e. when the current index mode is different to the one being applied. This prevents unnecessary trips to the database.

Also updates the messaging displayed to the admin to inform them how many indexers were updated and how many were skipped thanks to this change.

Finally, replaces the deprecated addSuccess() function with the non-deprecated addSuccessMessage() function for the message manager class when displaying the changed messages to the admin.

Manual testing scenarios (*)

1. Log into the admin
2. Set any number of indexers to be Update on Schedule
3. Observe the success message to confirm the correct number of indexers were updated versus skipped, based on their original mode

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)

[github-jira-sync-bot]

✅ Jira issue https://jira.corp.adobe.com/browse/AC-8076 is successfully created for this GitHub issue.

[m2-assistant]

✅ Confirmed by @engcom-Charlie. Thank you for verifying the issue.
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