* Note: breaking change; since this commit, a plugin that has
worked so far might get rejected due to plugin maturity
* mariabackup is not affected (allows all plugins)
* VERSION file defines SERVER_MATURITY, which defines the
corresponding numeric value as SERVER_MATURITY_LEVEL in
include/mysql_version.h
* The default value for 'plugin_maturity' is SERVER_MATURITY_LEVEL - 1
* Logs a warning if a plugin has maturity lower than
SERVER_MATURITY_LEVEL
* Tests suppress the plugin maturity warning
* Tests use --plugin-maturity=unknown by default so as not to fail
due to the stricter plugin maturity handling
