debian: choose mysql_native_password

In mysql version 8.x (*buntu 20.04), mythweb fails because
the PHP mysqli connector doesn't support the new default
caching_sha2_authentication. Existing DB users work OK,
but newly created ones won't.

Somewhat ugly solution as MySQL 5.x/8.x and MariaDB CREATE USER
syntax is slightly different. Use ALTER USER after creation.

See: https://mysqlserverteam.com/upgrading-to-mysql-8-0-default-authentication-plugin-considerations/
     https://mariadb.com/kb/en/create-user/#identified-viawith-authentication_plugin

Tested on MariaDB 10.3 and mysql 5.7 and 8.0.

Refs #13576

deb/debian/mythtv-database.postinst

@@ -37,8 +37,9 @@ ask_root_pw() {
 
 update_database() {
     #Set up privs for mythtv@network
-    if ! echo "CREATE USER IF NOT EXISTS $mythtv_username@'%' IDENTIFIED BY '$mythtv_password'; \
-               GRANT ALL ON $database.* TO $mythtv_username@'%';" | \
+    if ! echo "CREATE USER IF NOT EXISTS '$mythtv_username'@'%' IDENTIFIED BY '$mythtv_password'; \
+               ALTER USER '$mythtv_username'@'%' IDENTIFIED WITH mysql_native_password; \
+               GRANT ALL ON $database.* TO '$mythtv_username'@'%';" | \
         mysql $SECURITY_INFO "$database" >/dev/null 2>&1; then
         fail_database
     fi