Change Events API event retention to 30 days

[radville]

Why:

For https://github.com/github/sunset-squad/issues/33

Stratocaster now only stores data for 30 days.

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

This changes the doc about the Events API to indicate that data is now only shown from the last 30 days. It used to be 90 but was recently reduced to 30.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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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 the review server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
rest/activity/events.md	fpt ghec ghes@ 3.16 3.15 3.14 3.13 3.12	fpt ghec ghes@ 3.16 3.15 3.14 3.13 3.12

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

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[radville]

cc @github/sunset-squad-reviewers