PT-2340 - Support MySQL 8.4

- Adjusted sandbox scripts, so they can start MySQL 8.4
- Added MySQL 8.4 configuration file
- Removed empty file sandbox/5.6
- Removed unused files sandbox/set-mysql, sandbox/slave_channels_t.sql, sandbox/jenkins-test
- Removed offensive terminology from the sandbox scripts wherever it is possible

t/pt-archiver/archive_using_channels.t

@@ -91,10 +91,10 @@ like (
     'Message saying channel name must be specified'
 );
 
-push @args, ('--channel', 'masterchan1');
+push @args, ('--channel', 'sourcechan1');
 
 output(
-   sub { $exit_status = pt_archiver::main(@args, '--channel', 'masterchan1') },
+   sub { $exit_status = pt_archiver::main(@args, '--channel', 'sourcechan1') },
    stderr => 1,
 );