Change the collation in INFORMATION_SCHEMA to utf8mb4_general1400_as_ci

In order to prepare to MDEV-27490 let's change the collation in information_schema tables from utf8mb3_general_ci to utf8mb4_general1400_as_ci.

Changing the collation to utf8mb4_general_ci won't work, because it compares all supplementary character as equal.

So for example, I_S.COLUMNS will turn into:

CREATE TEMPORARY TABLE `COLUMNS` (

  `TABLE_CATALOG` varchar(512) NOT NULL,

  `TABLE_SCHEMA` varchar(64) NOT NULL,

  `TABLE_NAME` varchar(64) NOT NULL,

  `COLUMN_NAME` varchar(64) NOT NULL,

  `ORDINAL_POSITION` bigint(21) unsigned NOT NULL,

...

) ENGINE=Aria DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general1400_as_ci PAGE_CHECKSUM=0;

Metadata functions like:

• DATABASE()
• USER()
• CURRENT_USER()
• CURRENT_ROLE()
• CHARSET()
• COLLATION()
• VERSION()

will also change their collations to utf8mb4_general1400_as_ci

So will string system variables.

TODO:
The code using Item_string_sys will possibly also be affected.