Fix PHP 8.1 deprecations

I want to propose some updates to reduce the number of PHP 8.1-deprecation warnings & improve code quality:

In icegram/lite/classes/ig-mobile-detect.php, the match-function should be:

public function match($regex, $userAgent = null)
        {
            // In PHP 8.1+, passing null as subject to preg_match is deprecated.
            $subject = (false === empty($userAgent)) ? $userAgent : $this->userAgent;

            if (null === $subject) {
                return false;
            }

            $match = (bool) preg_match(sprintf('#%s#is', $regex), $subject, $matches);
            // If positive match is found, store the results for debug.
            if ($match) {
                $this->matchingRegex = $regex;
                $this->matchesArray = $matches;
            }

            return $match;
        }

in Icegram/lite/class-icegram.php, the icegram_load_data()-function should be:

		// Do not index Icegram campaigns / messages...
		// Not using currently - made custom post types non public...
		function icegram_load_data() {
			global $post;

			$sanitized_get_data = array();

			// FILTER_SANITIZE_STRING is deprecated in recent PHP versions.
			// Use a callback to sanitize each value instead.
			$get_data = filter_input_array( INPUT_GET, FILTER_DEFAULT );

			if ( ! empty( $get_data ) && is_array( $get_data ) ) {
				$sanitized_get_data = array_map(
					static function( $value ) {
						if ( is_array( $value ) ) {
							return array_map( 'sanitize_text_field', $value );
						}

						return is_scalar( $value ) ? sanitize_text_field( (string) $value ) : $value;
					},
					$get_data
				);
			}
			
			$icegram_pre_data['ajax_url']                        = admin_url( 'admin-ajax.php' );
			$icegram_pre_data['post_obj']                        = $sanitized_get_data;
			$icegram_pre_data['post_obj']['is_home']             = ( is_home() || is_front_page() ) ? true : false;
			$icegram_pre_data['post_obj']['page_id']             = is_object( $post ) && isset( $post->ID ) ? $post->ID : 0;
			$icegram_pre_data['post_obj']['action']              = 'ig_display_messages';
			$icegram_pre_data['post_obj']['shortcodes']          = $this->shortcode_instances;
			$icegram_pre_data['post_obj']['cache_compatibility'] = $this->cache_compatibility;
			$icegram_pre_data['post_obj']['device']              = $this->get_platform();
			
			wp_register_script( 'icegram_main_js', $this->plugin_url . '/assets/js/main.min.js', array( 'jquery' ), $this->version, true );
			if ( 'yes' === $this->cache_compatibility ) {
				if ( ! wp_script_is( 'icegram_main_js' ) ) {
					wp_enqueue_script( 'icegram_main_js' );
				}
			}
			wp_localize_script( 'icegram_main_js', 'icegram_pre_data', $icegram_pre_data );
		}

These both code-improvements will fix the issues with deprecated FILTER_SANITIZE_STRING and for using the “preg_match()”-function correctly.