MDEV-25829 Change default Unicode collation to uca1400_ai_ci

Step#2 - Adding a new collation derivation level for CAST and CONVERT.

Now character string cast functions:
  - CAST(string_expr AS CHAR)
  - CONVERT(expr USING charset_name)

have a new collation derivation level between:

  - string literals
  - utf8 metadata functions, e.g. user() and database()

Before the change these cast functions had collation derivation equal
to table columns, which caused more illegal mix of collation conflicts.

Note, binary string cast functions:
  - BINARY(expr)
  - CAST(string_expr AS BINARY)
  - CONVERT(expr USING binary)
did not change their collation derivation, to preserve the behaviour of
queries like these:
SELECT database()=BINARY'test';
SELECT user()=CAST('root' AS BINARY);
SELECT current_role()=CONVERT('role' USING binary);

Derivation levels after the change look as follows:

  DERIVATION_IGNORABLE= 7, // Explicit NULL

  DERIVATION_NUMERIC= 6,   // Numbers in string context,
                           // Numeric user variables
                           // CAST(numeric_expr AS CHAR)

  DERIVATION_COERCIBLE= 5, // Literals, string user variables

  DERIVATION_CAST= 4,      // CAST(string_expr AS CHAR),
                           // CONVERT(string_expr USING cs)

  DERIVATION_SYSCONST= 3,  // utf8 metadata functions, e.g. user(), database()
  DERIVATION_IMPLICIT= 2,  // Table columns, SP variables, BINARY(expr)
  DERIVATION_NONE= 1,      // A mix (e.g. CONCAT) of two differrent collations
  DERIVATION_EXPLICIT= 0   // An explicit COLLATE clause

mysql-test/main/func_misc.result

@@ -1477,10 +1477,10 @@ COERCIBILITY(NAME_CONST('name','test'))
 2
 SELECT COERCIBILITY(NAME_CONST('name',TIME'00:00:00'));
 COERCIBILITY(NAME_CONST('name',TIME'00:00:00'))
-5
+6
 SELECT COERCIBILITY(NAME_CONST('name',15));
 COERCIBILITY(NAME_CONST('name',15))
-5
+6
 SELECT CONCAT(NAME_CONST('name',15),'오');
 CONCAT(NAME_CONST('name',15),'오')
 15오