Update notification retention period

[bottsepp]

GitHub Copilot generated this pull request.

Summary

• Update the documented notification retention period from 5 months to 3 months.
• Apply the correction consistently to inbox, saved, and Done notification guidance.

Testing

• npm run lint-content -- --paths content/subscriptions-and-notifications/concepts/about-notifications.md content/subscriptions-and-notifications/get-started/configuring-notifications.md content/subscriptions-and-notifications/how-tos/viewing-and-triaging-notifications/triaging-a-single-notification.md content/subscriptions-and-notifications/how-tos/viewing-and-triaging-notifications/managing-notifications-from-your-inbox.md
• npm run build

Fixes github/notifications#7913

[github-actions]

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
subscriptions-and-notifications/concepts/about-notifications.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17
subscriptions-and-notifications/get-started/configuring-notifications.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17
subscriptions-and-notifications/how-tos/viewing-and-triaging-notifications/managing-notifications-from-your-inbox.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17
subscriptions-and-notifications/how-tos/viewing-and-triaging-notifications/triaging-a-single-notification.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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