Don't use .lower() to perform case-insensitive string operations

Use the faster versions with Qt::CaseInsensitive instead. These
don't require a string copy and conversion to lowercase.

Thanks to Clazy

src/core/processing/qgsprocessingparameters.cpp

@@ -888,7 +888,7 @@ bool QgsProcessingParameters::parseScriptCodeParameterOptions( const QString &co
 
   name = m.captured( 1 );
   QString tokens = m.captured( 2 );
-  if ( tokens.toLower().startsWith( QStringLiteral( "optional" ) ) )
+  if ( tokens.startsWith( QStringLiteral( "optional" ), Qt::CaseInsensitive ) )
   {
     isOptional = true;
     tokens.remove( 0, 8 ); // length "optional" = 8

src/core/qgsstacktrace.cpp

@@ -168,14 +168,14 @@ QgsStackTrace::QgsStackTrace()
 
 bool QgsStackTrace::StackLine::isQgisModule() const
 {
-  return moduleName.toLower().contains( "qgis" );
+  return moduleName.contains( "qgis", Qt::CaseInsensitive );
 }
 
 bool QgsStackTrace::StackLine::isValid() const
 {
-  return !( fileName.toLower().contains( "exe_common" ) ||
-            fileName.toLower().contains( "unknown" ) ||
-            lineNumber.toLower().contains( "unknown" ) );
+  return !( fileName.contains( "exe_common", Qt::CaseInsensitive ) ||
+            fileName.contains( "unknown", Qt::CaseInsensitive ) ||
+            lineNumber.contains( "unknown", Qt::CaseInsensitive ) );
 
 }
 ///@endcond