Microsoft has unveiled the next generation of its Windows software, called Windows 11, which has sleeker visual features and is more open to third-party apps. The newest version of Microsoft’s flagship operating system announced Thursday will be a successor to today’s Windows 10, which the company introduced in 2015. In a challenge to rival Apple, the company also announced that it won’t force app developers to pay fees to Microsoft for using its app store; and that Google’s popular Android apps will run on its new system. Windows 11 is expected to become available later this year on new computers and other devices and as a free update for those with Windows 10. It includes a host of cosmetic upgrades, such as a new Start button, a revamped taskbar and sounds, and under-the-hood features designed to boost speed and efficiency. Microsoft’s virtual announcement event was affected by technical difficulties, forcing …