Improve timezone setting logic

Previously, core.php tried to set the default timezone to the value
stored in global configuration, with error suppression. Due to [[1]],
this does not actually prevent the notice from appearing with PHP 8.

Refactored the code to get the system's default time zone if it is not
defined in configuration. A PHP notice is still thrown if the specified
timezone is not valid, as documented in the code.

Also, since PHP 5.4, date_default_timezone_get() no longer throws a
warning so the '@' operator is not needed anymore.

Fixes #27796

[1]: https://bugs.php.net/bug.php?id=80548

core.php

@@ -269,9 +269,12 @@ function autoload_mantis( $p_class ) {
 # Set the default timezone
 # To reduce overhead, we assume that the timezone configuration is valid,
 # i.e. it exists in timezone_identifiers_list(). If not, a PHP NOTICE will
-# be raised. Use admin checks to validate configuration.
-@date_default_timezone_set( config_get_global( 'default_timezone' ) );
-$t_tz = @date_default_timezone_get();
+# be raised and we fall back to the system's default timezone.
+# Use admin checks to validate configuration.
+$t_tz = config_get_global( 'default_timezone' );
+if( empty( $t_tz ) || !date_default_timezone_set( $t_tz )) {
+	$t_tz = date_default_timezone_get();
+}
 config_set_global( 'default_timezone', $t_tz, true );
 
 if( !defined( 'MANTIS_MAINTENANCE_MODE' ) ) {