rename plugin to "Statify Filter"

The plugins purpose is to exclude or filter certain requests from
tracking by Statify. However the current name "Statify Blacklist" is not
actually inline with today's understanding of such terms. In WordPress
Core 5.5 the word "blacklist" among others has been replaced by more
precise wording where possible.

The term "filter" has been used in various places already and clearly
describes the behavior of this plugin. So we rename the plugin to
"Statify Filter" and rephrase front-end texts.

Plugin slug (permalink), textdomain and all public class and constant
names left untouched for now, to not introduce breaking changes at this
point. To be cleaned up with next major release.

inc/class-statifyblacklist-system.php

@@ -1,6 +1,6 @@
 <?php
 /**
- * Statify Blacklist: StatifyBlacklist_System class
+ * Statify Filter: StatifyBlacklist_System class
  *
  * This file contains the derived class for the plugin's system operations.
  *
@@ -15,7 +15,7 @@ if ( ! defined( 'ABSPATH' ) ) {
 }
 
 /**
- * Statify Blacklist system configuration.
+ * Statify Filter system configuration.
  *
  * @since   1.0.0
  */