cmake-gui: Restore search bar case insensitivity

Since these changes:

* commit f1de6f6682 (cmake-gui: Support building against Qt6,
                     2020-12-17, v3.20.0-rc1~205^2~7)
* commit 134d69121e (cmake-gui: Restore search bar for cache view and
                     environment dialog, 2021-02-22, v3.20.0-rc2~12^2)

the search bar is no longer case insensitive when using Qt 5.12.[0-1],
and possibly some later versions.  It works as of Qt 5.15 though, so
update our version check accordingly.

Fixes: #21884

Source/QtDialog/EnvironmentDialog.cxx

@@ -81,7 +81,7 @@ bool EnvironmentSearchFilter::filterAcceptsRow(int row,
   auto* model = this->sourceModel();
   auto key =
     model->data(model->index(row, 0, parent), Qt::DisplayRole).toString();
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 12, 0))
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 15, 0))
   return key.contains(this->filterRegularExpression());
 #else
   return key.contains(this->filterRegExp());
@@ -110,7 +110,7 @@ EnvironmentDialog::EnvironmentDialog(const QProcessEnvironment& environment,
                    &EnvironmentDialog::addEntry);
   QObject::connect(this->RemoveEntry, &QAbstractButton::clicked, this,
                    &EnvironmentDialog::removeSelectedEntries);
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 12, 0))
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 15, 0))
   QObject::connect(this->Search, &QLineEdit::textChanged, this->m_filter,
                    QOverload<const QString&>::of(
                      &EnvironmentSearchFilter::setFilterRegularExpression));

Source/QtDialog/QCMakeCacheView.cxx

@@ -47,7 +47,7 @@ protected:
 
     // check all strings for a match
     foreach (QString const& str, strs) {
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 12, 0))
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 15, 0))
       if (str.contains(this->filterRegularExpression())) {
 #else
       if (str.contains(this->filterRegExp())) {
@@ -170,7 +170,7 @@ bool QCMakeCacheView::showAdvanced() const
 
 void QCMakeCacheView::setSearchFilter(const QString& s)
 {
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 12, 0))
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 15, 0))
   this->SearchFilter->setFilterRegularExpression(s);
 #else
   this->SearchFilter->setFilterFixedString(s);