Eliminate MutationObserver.js

[sivaschenko]

Remove lib/web/MutationObserver.js and replace usages with native JS MutationObserver.

Following the investigation: #32856

[m2-assistant]

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[anzin]

@magento I am working on this

[sivaschenko]

@magento export issue to Jira project AC as Story

[github-jira-sync-bot]

❌ Cannot export the issue. This GitHub issue is already linked to Jira issue(s): https://jira.corp.magento.com/browse/AC-247