restrict mysql_enable_utf8 to DBD::mysql

[maxbes]

Test script:

use Apache::Session::Browseable::MySQL;
  
my $args = {
        'DataSource' => 'DBI:MariaDB:database=test;host=localhost',
        'Password' => 'test',
        'TableName' => 'sessions',
        'UserName' => 'test',
};

tie %session, 'Apache::Session::Browseable::MySQL', undef, $args;

Version: 10.11.4-MariaDB

result:

DBD::MariaDB::db STORE failed: Unknown attribute mysql_enable_utf8

This PR only sets mysql_enable_utf8 on DBD::mysql. DBD::MariaDB already does it by default