Replace user-switching plugin in failing tests

[Pascal Birchler]

Bug Report

• Yes, I reviewed the contribution guidelines.
• Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Describe the current, buggy behavior

A few of the wp plugin * commands tests activate the User Switching plugin. The plugin recently bumped its minimum PHP version, which means those tests now fail.

Describe how other contributors can replicate this bug

Failing tests:

• Scenario: Activate all when one plugin is hidden by "all_plugins" filter # features/plugin-activate.feature:37
• Scenario: Return code is 1 when one or more plugin installations fail # features/plugin-install.feature:113
• Scenario: Verify installed plugin activation # features/plugin-install.feature:222
• Scenario: Plugin hidden by "all_plugins" filter # features/plugin.feature:577

Example run: https://github.com/wp-cli/extension-command/actions/runs/6914340823/job/18813033559?pr=386

Describe what you would expect as the correct outcome

Let's use another common plugin that still supports PHP 5.6 so that the tests pass again.