Mark converted_to_discussion issue event type as available in timeline API

[Fs00]

Why:

Closes: #35045

What's being changed (if available, include any code snippets, screenshots, or gifs):

The converted_to_discussion issue event type is now correctly reported as available in the timeline REST API.

Check off the following:

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline (this link will be available after opening the PR).

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

[github-actions]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
rest/using-the-rest-api/issue-event-types.md	fpt ghec ghes@ 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.14 3.13 3.12 3.11 3.10

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

[nguyenalex836]

@Fs00 Thank for opening a PR with this fix! I'll get this merged 🍏

[github-actions]

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