Add milestoned and demilestoned to pull_request_target activity types

[Elqudsy]

Summary

Adds milestoned and demilestoned to the pull_request_target activity types table in the GHES < 3.20 section, matching the pull_request table.

Closes #42851

Why

The pull_request_target table was missing these two events, making it out of sync with the pull_request table and with the recent changes in actions/languageservices#242.

[github-actions]

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Table of review links

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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
actions/reference/workflows-and-actions/events-that-trigger-workflows.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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[subatoi]

Hi–I can't see the reasoning behind this, so I'm going to go ahead and close this out. Please feel free to open an issue with a description, if you've identified a discrepancy. Thanks