MDEV-12501 -- set --maturity-level by default

* 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

mysql-test/include/default_mysqld.cnf

@@ -17,7 +17,7 @@
 # Default values that applies to all MySQL Servers
 [mysqld]
 disable-getopt-prefix-matching
-
+plugin-maturity=unknown
 open-files-limit=           1024
 local-infile
 character-set-server=      latin1

mysql-test/include/have_plugin_auth.opt

@@ -1 +1 @@
---plugin-load-add=$AUTH_TEST_PLUGIN_SO
+--plugin-load-add=$AUTH_TEST_PLUGIN_SO --plugin-maturity=unknown

mysql-test/include/mtr_warnings.sql

@@ -224,6 +224,12 @@ INSERT INTO global_suppressions VALUES
  ("Slave I/O: Setting @slave_gtid_ignore_duplicates failed with error.*"),
  ("Slave I/O: Setting @slave_until_gtid failed with error.*"),
  ("Slave I/O: Get master GTID position failed with error.*"),
+
+ /*
+   MDEV-12501 -- set --maturity-level by default
+ */
+ ("Plugin .* is of maturity level .* while the server is .*"),
+
  ("THE_LAST_SUPPRESSION")||