Update Qt to 5.15.15 on Windows.

Telegram/build/prepare/prepare.py

@@ -457,7 +457,7 @@ if customRunCommand:
 stage('patches', """
     git clone https://github.com/desktop-app/patches.git
     cd patches
-    git checkout 85a1c4ec32
+    git checkout ac72a4f597
 """)
 
 stage('msys64', """

Telegram/build/qt_version.py

@@ -9,7 +9,7 @@ def resolve(arch):
             os.environ['QT'] = '6.8.0'
         elif os.environ.get('QT') is None:
             print('Choosing Qt 5.')
-            os.environ['QT'] = '5.15.13'
+            os.environ['QT'] = '5.15.15'
     elif os.environ.get('QT') is None:
         return False
     print('Choosing Qt ' + os.environ.get('QT'))