Update Dependabot options reference for pull request limits

[FaithOmbongi]

Why:

Closes: #46116

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

• Fix note regarding open PRs for security updates.
• Link to error reference code for when open-pull-request-limit (explicit or default) is reached.

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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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
code-security/reference/supply-chain-security/dependabot-options-reference.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18

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

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[mchammer01]

@FaithOmbongi👋🏻 - thanks so much for digging into this and for the clear writeup in the issue! ✨
This looks great: accurate, clear, and links to the troubleshooting guide nicely. Approving! 🙌🏻

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