Remove hardcoded default collation for MySQL tables

Let MySQL/MariaDB use the database default collation instead of
forcing utf8mb4_unicode_ci on every table. This avoids collation
mismatches on servers with different defaults (e.g. MariaDB 11.8
uses utf8mb4_uca1400_ai_ci).

Users who need a specific collation can still set it explicitly
in their migrations.

Fixes #1010

Author: dereuromark

docs/en/writing-migrations.rst

@@ -519,10 +519,9 @@ In addition, the MySQL adapter supports following options:
 Option     Platform         Description
 ========== ================ ===========
 comment    MySQL, Postgres  set a text comment on the table
-collation  MySQL, SqlServer the default collation for a table if different than the database.
+collation  MySQL, SqlServer set the table collation *(defaults to database collation)*
 row_format MySQL            set the table row format
 engine     MySQL            define table engine *(defaults to ``InnoDB``)*
-collation  MySQL            define table collation *(defaults to ``utf8mb4_unicode_ci``)*
 signed     MySQL            whether the primary key is ``signed``  *(defaults to ``false``)*
 limit      MySQL            set the maximum length for the primary key
 ========== ================ ===========

src/Db/Adapter/MysqlAdapter.php

@@ -167,7 +167,6 @@ public function createTable(Table $table, array $columns = [], array $indexes =
         // This method is based on the MySQL docs here: https://dev.mysql.com/doc/refman/5.1/en/create-index.html
         $defaultOptions = [
             'engine' => 'InnoDB',
-            'collation' => 'utf8mb4_unicode_ci',
         ];
 
         $options = array_merge(