Fix multisite activation

[mauteri]

Description of the Change

Closes #

How to test the Change

Changelog Entry

Added - New feature
Changed - Existing functionality
Deprecated - Soon-to-be removed feature
Removed - Feature
Fixed - Bug fix
Security - Vulnerability

Credits

Props @username, @username2, ...

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[what-the-diff]

PR Summary

• Modified class-setup.php file
  This pull request primarily represents modifications in class-setup.php file to optimize code readability and functionality.

• Renaming of methods
  Key methods like maybe_flush_gatherpress_rewrite_rules and maybe_create_custom_table have been renamed to maybe_flush_rewrite_rules and create_tables respectively for greater clarity.

• Creation of New Methods
  There are new methods such as on_site_create and maybe_create_flush_rewrite_rules_flag. The on_site_create method manages actions linked to the creation of a new site in a multisite network, while maybe_create_flush_rewrite_rules_flag manages the creation of an option flag that determines whether to execute a process called 'rewrite rules'.

• Readme.md file update
  The readme.md file has been updated to show that our platform is now compatible with GatherPress WordPress Multisite.

• New unit test
  An additional unit test for maybe_create_flush_rewrite_rules_flag method has been implemented to ensure correctness of the new functionality introduced.