Stringify variables names for mysql connections

For mysql2/mysql adapters, `sql_mode` variable name set in `database.yml`
as string, was ignored and `sql_mode` was set to use strict mode.

Fixes #14895
Merge pull request #14896 from jetthoughts/14895_overide_strict_by_sql_mode

Symbolize variables of mysql/mysql2 connection configuration

Conflicts:
	activerecord/CHANGELOG.md