Explicitly mention SMS as insecure method

[RCheesley]

Why:

When completing some security checks, we were having multiple team members flagged up as 'insecure' despite the fact that they had many different secure methods enabled in their settings. I was not able to find anywhere what might be causing the insecure flag to be applied, because it isn't explicitly called out in the docs anywhere. It's mentioned in passing here:

https://github.blog/changelog/2024-11-21-enhanced-2fa-management-for-orgs-and-enterprises-public-preview/

Currently, GitHub defines SMS/text message as an insecure method of 2FA, and TOTP authentication applications, the GitHub Mobile app, security keys, and passkeys as secure methods.

This PR adds a specific reference to SMS being an insecure method.

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

It just adds an explicit mention that SMS is considered an insecure method.

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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Source	Review	Production	What Changed
organizations/keeping-your-organization-secure/managing-two-factor-authentication-for-your-organization/requiring-two-factor-authentication-in-your-organization.md	fpt ghec ghes@ 3.17 3.16 3.15 3.14 3.13	fpt ghec ghes@ 3.17 3.16 3.15 3.14 3.13

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

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[Sharra-writes]

Thanks for opening a PR! This is definitely a good change. I just need to check our style guide to make sure the information is being added in the correct format, and then I'll either update the format or proceed with getting the PR merged.

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