keepassxc: Update to 2.6.4

Changelog:
2.6.4 Changelog

Added

  * Automatically adapt to light/dark system theme changes (Windows/macOS only)
    #6034

Changed

  * Show window title as tooltip on system tray #5948
  * Compress Snap release as LZO for faster initial startup #5877
  * Password generator: Set maximum selectable password length to 999 #5937

Fixed

  * Fix crash on app close when using SSH agent #5935
  * Fix KDF selection showing wrong item when using Argon2id #5923
  * Automatically close About dialog on database lock if it is still open #5947
  * Linux: Fix automatic launch at system startup with AppImages #5901
  * Linux: Fix click-to-move on empty area activating when using menus #5971
  * Linux: Try multiple times to show tray icon if tray is not ready yet #5948
  * macOS: Fix KeePassXC blocking clean shutdown #6002

(ryoon)