Fix MantisCore plugin dependencies

In issue #17360, we introduced setting a default maximum version for
MantisCore to avoid compatibility issues by disabling plugins which have
not been updated for a new Mantis release, with a break on every minor
version (x.Y.z).

With 2.0 and the decision to only introduce breaking changes on major
versions (X.y.z), this code has to be changed.

Fixes #22171

core/plugin_api.php

@@ -494,38 +494,20 @@ function plugin_dependency( $p_base_name, $p_required, $p_initialized = false )
 
 		$t_required_array = explode( ',', $p_required );
 
-		# If the plugin's minimum dependency for MantisCore is unspecified or
-		# lower than the current release (i.e. the plugin does not specifically
-		# list the current core version as supported) and the plugin does not
-		# define a maximum dependency, we add one with the current version's
-		# minor release (i.e. for 1.3.1 we would add '<1.3').
-		# The purpose of this is to avoid compatibility issues by disabling
-		# plugins which have not been updated for a new Mantis release; authors
-		# have to revise their code (if necessary), and release a new version
-		# of the plugin with updated dependencies.
-		# To indicate compatibility (e.g. with release 1.3), one can either:
-		# 1. update the minimum required version (i.e. plugin only works with
-		#    1.3; 1.2-compatible code is maintained separately):
-		#    $this->requires = array( 'MantisCore' => '1.3' );
-		# 2. add the new release as a 2nd minimum version (i.e. the plugin is
-		#    compatible with both versions):
-		#    $this->requires = array( 'MantisCore' => '1.2, 1.3' );
-		# 3. add a maximum version higher than the new release:
-		#    $this->requires = array( 'MantisCore' => '1.2, <2.0' );
-		#    Note that this may cause the plugin to face compatibility issues
-		#    if and when a version 1.4 is released.
+		# Set maximum dependency for MantisCore if none is specified.
+		# This effectively disables plugins which have not been specifically
+		# designed for a new major Mantis release to force authors to review
+		# their code, adapt it if necessary, and release a new version of the
+		# plugin with updated dependencies.
 		if( $p_base_name == 'MantisCore' && strpos( $p_required, '<' ) === false ) {
-			$t_version_core = substr(
-				$t_plugin_version,
-				0,
-				strpos( $t_plugin_version, '.', strpos( $t_plugin_version, '.' ) + 1 )
-			);
+			$t_version_core = utf8_substr( $t_plugin_version, 0, strpos( $t_plugin_version, '.' ) );
 			$t_is_current_core_supported = false;
 			foreach( $t_required_array as $t_version_required ) {
 				$t_is_current_core_supported = $t_is_current_core_supported
 					|| version_compare( trim( $t_version_required ), $t_version_core, '>=' );
 			}
 			if( !$t_is_current_core_supported ) {
+				# Add current major version as maximum
 				$t_required_array[] = '<' . $t_version_core;
 			}
 		}