Step#1 - Changing collation derivation for string user variables
from IMPLICIT to COERCIBLE.

Retionale:

Without this preparatory change, switching the default collation for
Unicode character sets from xxx_general_ci to uca1400_ai_ci would cause
"Illegal mix of collations" errors in scenarios comparing a column with
a non-default collation to a string user variable

This is especially important for queries to INFORMATION_SCHEMA tables,
whose columns use utf8mb3_general_ci.

See the description of MDEV-25829 for more details and SQL script examples.