PHP 7.4 Compatibility

[kevinfodness]

An issue to collect problems with PHP 7.4 and the current codebase.

• Warning in bin/wp-cli.php: Method name Searchpress_CLI_Command::__apply_date_range is discouraged; PHP has reserved all method names with a double underscore prefix for future use.

[renatonascalves]

This looks like a duplicate of #105, which already has a pull request.

[mogmarsh]

handled by https://github.com/alleyinteractive/searchpress/commit/14b34ee531b04efd119c109e4faff409319fc002