Use X instead of Twitter for social buttons

[alanorth]

Description

The default configuration and tests use twitter instead of x. Twitter was rebranded as X in 2023 so we should not be using that name anywhere.

Instructions for Reviewers

Please add a more detailed description of the changes made by your PR. At a minimum, providing a bulleted list of changes in your PR is helpful to reviewers.

List of changes in this PR:

• First, update example config to use x instead of twitter for social sharing icons
• Second, update test configuration to use x instead of twitter

Include guidance for how to test or review your PR. This may include: steps to reproduce a bug, screenshots or description of a new feature, or reasons behind specific changes.

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