Make example keyset pagination header parsing consistent

[PauwelsPieter]

Why: Inconsistent use of uppercasing when parsing the link headers in documentation.

Closes: /

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

Link to the specific [section in the docs](https://docs.github.com/en/rest/using-the-rest-api/using-pagination-in-the-rest-api?apiVersion=2022-11-28#example-creating-a-pagination-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.

[github-actions]

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Source	Review	Production	What Changed
rest/using-the-rest-api/using-pagination-in-the-rest-api.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14

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

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[Copilot]

Pull request overview

This PR fixes an inconsistency in the keyset pagination example code where the regex pattern for parsing HTTP Link headers was using uppercase "Next" instead of the correct lowercase "next". According to RFC 8288 (Web Linking), relation types in Link headers are case-insensitive but conventionally lowercase. The fix ensures the regex pattern at line 130 now matches the lowercase "next" already used in the string check at line 148, making the example code consistent and following the standard convention.

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[Sharra-writes]

@PauwelsPieter This change seems fine, so 🤞 that the CI checks decide to work. There's one that's not passing for pretty much everyone that I can ignore, but I'm getting another occasionally being weird now.

[github-actions]

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